Back to The Future/WMG

In 1955, Biff doesn't live with his grandmother.
He lives with his step-mother, Edna. He just calls her "grandma" as an insult.
 * This is in violation of the first rule of BTTF time travel: you can never see the effects of your own time travels before actually performing those time travels.
 * The whole premise of Back To The Future Part II is a violation of this rule, because Marty and Jennifer see their children despite, from an external perspective, having just disappeared for 30 years. They should not be able to see their children because they haven't travelled back in time to 1985 yet, nor got old enough to finish school, get married and have those kids in the first place.
 * No No NO! That only applies if they had opted to stay in the future.
 * Perhaps the 2015 they visit is the version of history wherein they hadn't, as teenagers, gone forward in time to 2015. Here's how it works: After surviving the bullets in the new version of 1985 at the end of Part 1, Doc decides to go forward in history. He does completely disappear from the timeline (like the previous point argues that Marty and Jennifer should have) until 2015, and everything else happens "normally" until that point. For example, the car accident that we see not happen at the end of Part 3 does. (Heck, perhaps middle-aged-Marty's psychological problems are also partly related to the trauma of Doc having disappeared.) Doc reads the newspaper article about Marty Jr in jail, and decides he must prevent it. He goes back to 1985 and picks up Jennifer and Marty, then goes forward. This would cause timeline alteration, including a no-Jennifer-and-Marty universe, except that the ripple effect "hasn't happened yet", in accordance with BTTF metaphysics. So they necessarily visit the exact same 2015 as the one from which Doc departed. It is only afterwards that things diverge (with the usual weird risks of paradoxes and whatnot). The same principle seems to hold with old-Biff's adventures — he goes back in time and gives the almanac, which means that when he goes forwards, one would think he would arrive in a rich-Biff timeline, but he doesn't because (his having left 1955 prior to the branching point at which Marty either does or doesn't successfully take the almanac back) the changes haven't propogated forward "yet". Sort of. (He has the heart attack because the rich-Biff ending up dying in middle age.)

The Delorean door opening in Part II killed Biff.
In a deleted scene, we see old Biff dissappear after. that's because in A!1985, The door opening killed A!Biff, so Old biff didn't exist.
 * Jossed by Word of God: DVD special features for Part II state that Biff was killed in 1996-A, most likely by

Doc is a Chessmaster who planned out the entirety of Part II and III to ensure a better future for Marty
At the very end of the third movie, when Doc comes back in the train with his kids, Jennifer asks him about the fax, and the fact that it erased. He says of course it did, the future isn't written yet you can make it whatever you please. In my mind, this kinda cheapened the entire second movie. Why bother going to the future to change what happened to Marty Jr if the future isn't set in stone and can be changed?

But...it wasn't pointless. Doc PLANNED the entire thing. Doc knew that Marty being a hothead about being called "chicken" resulted in a future that was...not really great. He wants Marty to get over this, but can't just say "You're going to be in a car race that Needles eggs you in to, you're going to hit a Rolls Royce and ruin your future." Doing that would cause problems, because as he says multiple times, no one should know too much about their own future. So instead, he comes up with a plan: set into action a chain of events that will cause Marty to mature, and get over his whole problem with being called chicken. So he uses his knowledge of how timelines and time travel works, and figures out this plan, that involves the entirety of Btt F 2 AND 3, that eventually gets Marty to mature. His tombstone just HAPPENS to be next to where the De Lorean is buried in 1955? No...he planned it that way. He let it be known that is where he wanted to be buried, so that Marty would see the tombstone in 1955 and plan to come back to the past, and that the events of part 3 would happen and allow Marty to get over his issues and not get in that accident. Another part of the theory is the malfunctioning Time Circuits in part 2. They worked just fine, he simply made it look like they weren't so he'd have an excuse to be sent back to 1885. When he was trying to land the De Lorean in 1955 after Marty burned the book, he says that he has to fly back around due to the wind, but in what we've seen of the De Lorean flying previously, it seems like it is able to do a vertical landing, akin to a harrier jet. So why suddenly does the wind matter? It doesn't. Doc knows that if he stays in the air he will get hit by lightning, and has the time circuits ready to go to 1885, as it is all part of his perfectly thought-out plan.

Doc is a total bad-ass, and one of the greatest Chessmasters ever committed to film, using his own death as part of his plan.


 * "Why bother going to the future to change what happened to Marty Jr if the future isn't set in stone and can be changed?"

In a way, this question answers itself. Because the future can change… one may as well "bother" to change the future. It just happens that Doc is cautious enough to not want to change the future "too much", so he doesn't try to prevent the Marty-Jr-in-jail timeline by simply warning young Marty about his future son and waiting thirty years.

Apart from that, this is very clever, though of course like nearly any Xanatos Gambit in bot film and real life, there seem to be many many ways it could have gone wrong.

Doc's inventions created the futuristic 2015 of the movie.
The reason why the 2015 in the movie looks nothing like what the true year of 2015 will be is because we're in a reality with no Doc Brown. The man is a genius who is WAY ahead of his time and it's fairly likely that he'd be capable of many other inventions after mastering time travel. It's possible then that everything from hoverboards to hovercars and anything else you saw in that futuristic 2015 were all based on Doc Brown's prototypes. Still odd how cell phones or laptops don't exist in that reality though.
 * Cell phones and laptops may very well be children's toys in that reality.

The DeLorean's LED readout indicating the year is broken.
Specifically, the space which records the last digit of the year, which is why they can only travel to years ending in "5". (The time when Doc demonstrated the display with the years "1776" and "0000" was before it broke.)
 * Considering that there is no Year 0...yeah, it was probably broken then, too. It may have gotten broken when Doc entered "Dec 25 0000."
 * Of course there is a year zero, just not everyone calls it that. The number assigned to a year is an arbitrary convenience. It doesn't matter what label your calendar slaps on it, if the time machine knows to go back a certain number of years that's all that matters. Doc programmed his Delorean with a different calendar than the common modified Gregorian-- there are several calendars that use a zero year to mark the birth of Christ.
 * Maybe the necessary equations involve dividing by the year.
 * Maybe that's how the Doc broke it (by setting it to zero)
 * ...yeaaaah, uh, The Telltale games called; they say this is Jossed.
 * The simple explanation is that the DeLorean LED readout is merely a relative measurement. i.e. going to 0000 from anywhen AD would result in landing in -1 AD/1 BC

Doc Brown is The Doctor.
An eccentric old man, more then happy to take an impulsive teenager back and forwards in time with him, but still takes preventing paradox's very seriously. Pacifistic enough that knowing a group of Libyan terrorists is coming after him he uses the information to get a Kevlar vest, which would do nothing if the terrorists decided to shoot him in the head point blank, even though he could have easily set up a trap to kill them. He must have aged at least 35-40 years (based on estimated age of the kids on the train) between 1955 and the end of film yet he looks no older.there are about 200 years between episode 11 and 12 of doctor who season 6 during this time we know nothing about his whereabouts. alternatively it is made very clear that the doctors intention at the end of season 6 is to lay low for a while. oh and his nickname is doc. just saying.
 * Either that, or Doc Brown is the Doctor after his twelve regen limit is used up, and he's just living through the rest of the years. However, it is also possible that Doc Brown is just a human who was visited by the Doctor. Doc had a vision, and the Doctor just happened to take a liking to him--he "gently nudged" Doc in the right direction. On top of all that, the De Lorean has to use the Time/Space Vortex in some way: one miscalculation, and your bright, shiny Time Machine goes flying off the face of the earth.

The Flux Capacitor doesn't make Time Travel itself possible, it makes it possible for a person.
The real function of the Flux Capacitor is to ensure that anyone traveling through time remains isolated from changes made to the time stream that may affect them. This is why Marty retains his memories from the "original" timeline and his Ripple Effect proof memory. Doc doesn't actually use the machine himself until the very end of the film and so his past was changed thanks to Future!Marty's involvement while Marty remembers things the way the used to be. This is also why Marty fades from time so slowly. In reality he should be the first kid in his family to fade because if they never married and had Dave they sure as hell aren't going to have Marty, but Marty was shielded from time's effects due to the Flux Capacitor.

The DHARMA Initiative hired the Libyans to assassinate Doc Brown
In a bizarre coincidence, the VW bus the Libyans drive is the exact same model and color as the kind DHARMA uses all over the island. Because rogue time travelers would cause DHARMA quite a few problems, they sent the Libyans to assassinate anyone who might time travel in hopes of stopping future problems.
 * They attempted to steal back the plutonium because they feared it was the origin of the hydrogen bomb used on the island by the time travelers in 1977. Because the Others hid Jughead underground, DHARMA never knew where it came from, and thus assumed future time travelers Marty and Doc were the source of the nuclear material.
 * The only problem is the Celebrity Paradox: Back to the Future exists in Lost's universe; one episode has Hurley checking his hand to see if he's disappearing and he references the franchise's time travel rules.

At some point between reacting to the "new" 1985 and arriving in 2015, Marty's memories and personality change to fit the new version of history.
First-movie!Marty has self-esteem issues so bad, he won't send his demo tape to a record producer in case they don't like it. Second-movie!Marty automatically assumes he's going to be a famous rock star. This discrepancy is because second-movie!Marty was influenced by a father who was a successful author, not one who never sent his book to a publisher for exactly the same reason Marty wouldn't send his demo tape. The new Marty's improved self-image has a downside, however: that sensitivity to being called "chicken."
 * This would also explain why Doc and Marty acted as though the new 1985 was the "correct" timeline throughout Part II. It just took a while for the Delayed Ripple Effect to get to Marty's memories.
 * It may also explain where Marty's weakness to the word "chicken" came from. Something may have happened exclusively in the new timeline to cause his disposition, explaining why this was absent from the first film.
 * To be fair, it was the correct timeline for Doc. Only Marty traveled back in time at that point.
 * Perhaps it's the same incident which made Needles his arch-nemesis who was never mentioned in the first film?
 * This is actually Fridge Brilliance, rather than Wild Mass Guessing. Otherwise, the "chicken" issue becomes something close to absurdity.
 * It also explains why the new 1985 is a good thing rather than the Fridge Horror of your family of 17 years having different personalities and histories--eventually, Marty acclimated to those changes as his new memories caught up.
 * Second-movie!"1955"!Marty is taking part in what appears to him as a Stable Time Loop. The ripple effect went round and round until it found an equilibrium.

The McFly family is somewhat inbred.
Marty's paternal great-great-grandmother looks just like his mother. George and Lorraine are second cousins, although they are unaware of it.

Marty and the Doc are the same person.
The Doc never seems to age visibly; could he perhaps be an older (much older) version of Marty who has come across or invented some kind of rejuvenation treatment?
 * In Part III, Doc tells Marty a bit about his childhood, which would be pointless if they were the same person. Marty doesn't seem like the type of kid who would be enthralled by Jules Verne anyway. Furthermore, the Doc briefly touches on his family's history in the same movie, and he mentioned in the first film that it took his entire family fortune to build the time machine; therefore, he definitely has a background.
 * It is established in film continuity that Doc Brown is very against letting your earlier self become aware that you are their later self! He'd lie to Marty about their being the same person to preserve the universe.
 * We see the Jules Verne bits when he's with Clara. He has no reason to lie to her.
 * Women talk, man. Clara could accidentally let something slip to Marty. Either that, or Doc replaced his real memories as of being Marty McFly with false memories of a past as Emmett Brown to completely make himself a different person as seen below.
 * Doc is visibly younger in 1955 than in 1985 in the first movie. After going to the future, he got a rejuvenation that allowed the actor to play 1985-Doc without heavy age makeup. Granted, at the end of Part 3 about ten years have passed for Doc, but maybe rejuvenation slows senescence.
 * Okay, what are you smoking what version of the film did you watch?
 * The one where Doc in part II wears a Latex Perfection mask to avoid giving away his age-- and when he pulls it off looks exactly the same as he does in all three films.
 * Marty went further into the future to not only get a rejuvenation, but to have his brain genetically enhanced to make himself a genius-- a side-effect was that it altered his personality. Before he went back and became Doc, he used his new intelligence to realize the damage he was doing to the timestream, and he had his memory erased or altered just before going back; he then remembered/"invented" the time machine.
 * Does it even need to be stated that the Telltale games Joss this three ways 'til Thursday?

Old Biff's reason for choosing November 12, 1955:
When Old Biff gave the Timeline-Altering MacGuffin to his younger self on the date of the lightning storm, it was no coincidence. Biff chose that date because he believed that that day was the day that his entire life changed. And it was. Getting decked by George turned him from a bully into a relatively nice guy a wimp afraid of his former victim. Old Biff decided that that life sucked, and he still had a grudge against George McFly, and so he went back to the moment that things changed.
 * There's a deleted scene from Part II where the Clock Tower Guy in 2015, Terry, reminds Old Biff of the date while shaking Marty down for money. Terry was the mechanic who told Biff it was 300 bucks for getting all that manure out of his car. Probably hearing about the manure helped remind Old Biff of all the other stuff, so yeah. Off to November 12th!

The Sims 3 is set in The Present Day (2008) version of Hill Valley.
Compare Hill Valley and The Sims 3
 * In The Sims, Sunset Valley/Pleasantview (the main setting of TS3 was founded by Gunther Goth, who is middle-aged in The Sims 3, the chronologicaly first game in the series. Hill Valley was founded in the year 1850 in BTTF, so that is impossible.
 * Sure, THAT'S what's impossible, in a shared universe in which physical time travel is possible via a sports car.

Seamus and Maggie McFly are Lorraine's great-grandparents, not George's.
The surname is just a coincidence. One of them looks like Lorraine, the other looks like Marty -- not George, Marty. Marty didn't get his looks from his father's side, but from his mother's. (When you think about it, he looks more like her anyway.) This also explains why George and Lorraine are still happily married despite the implications of her having a son who looks exactly like her high-school boyfriend Calvin. She has photographic proof that that face runs in her family.
 * It makes more sense that George has photographic proof that Marty's face runs in HIS family.
 * See also the inbreeding theory above.
 * Of course, there is William "good-looking guy" McFly.
 * Utterly and thoroughly Jossed in the Telltale games, where we meet an adult William and Arthur McFly; the former indicates that Seamus was his father and Arthur is his son. And George indicates that Arthur was his father (which, given that he's an Identical Son, didn't really need to be stated, but still...)

Doc got the time machine from the future.
He came up with the Flux Capacitor concept when he hit his head on the sink and was unconscious, right? Well, maybe Future!Doc (or someone) came back and implanted the idea in his head using future neural technology, or old-fashioned hypnosis? The time machine has no origin, it just keeps going around and around...
 * He must have enlisted the help of Leonardo Di Caprio.
 * The time machine still needs an origin.
 * It doesn't.
 * Consider that, in the BTTF universe, Johnny B Goode doesn't have an actual origin.
 * Face Palm BTTF is not a Stable Time Loop, as proven by all the changes like George's assertiveness. And even "Johnny B. Goode" is a terrible example because Chuck did not hear anywhere near enough of the song to have stolen it, and Marvin had no opportunity to memorise the lyrics he did hear.
 * Consider that by the time Part III happened, Doc learned that the Flux Capacitor could actually work, he'd already had a week to study the original time machine, and even rebuilt the Time Circuit Control microchip with 1955 components. All that he really needed was to wait until 1981 (When the De Lorean DMC-12 was released) to start building it.

Marty's "daughter" is a trans-gendered son.
Yeah, Michael J. Fox played the part, but seriously, what woman has gams like that? It's never brought up on-screen because, well, it's the future, and such things are no longer an issue.
 * "it's the future"?? The "future" of Back to the Future is only a shade over 5 years from now. (Feel old yet?)
 * WHERE'S MY HOVERBOARD?!
 * Here.
 * What I meant that the the 2015 they went to is The Future (TM), not necessary just five years (well, four years now), in our future.

The father of the family that lives in Marty's house in 1985-A is Samuel L Jackson.
He looks (and sort of sounds) like he's played by Samuel L. Jackson, but he was played by Al White. Instead, the character (only listed as "Dad") is Samuel L. Jackson.

One of the effects of 1985-A is that Samuel L. Jackson's movie career was unsuccessful, probably due to Biff's influence over Hollywood. We know that Biff lobbied in the state capitol and was helping Richard Nixon get elected to a fifth term in office; it's reasonable that he also had some sway in Hollywood. Thus, Samuel L. Jackson settled into a quiet suburban life in Hill Valley.
 * I always thought it was Goldie Wilson, who never got to be mayor due to Biff's influence.
 * That would actually make some dramatic sense.

1985-A is the same 1985 that Watchmen takes place during.
There is a lot of crime, and Nixon is president. And it would be cool. meant to be present in this stage in time?
 * You think a being like Dr. Manhattan wouldn't have something to say about a figure like Biff using an object clearly not
 * It may have something to do with Doc M being created some four years after Biff changed the future.
 * But, the headline in 1985-A shows that the Vietnam War was still going on until at least 1983. Whereas in Watchmen, the war was resolved with a quick victory for America.
 * Could be a different Vietnam War. In Watchmen, the US and Russia are implied to have extremely aggressive militaries.
 * Doc's family did change their name some time in the past, but in one timeline it became Brown, in another it was Osterman. The split occurred when Manhattan went back and caused his family to settle in California instead of New York, in hopes of preventing his father from becoming a watchmaker.

Marty jumped to his death during 1985-A
When Biff traps Marty on the roof, Doc seems to be waiting for him rather conveniently. It seems likely that Marty died in one timeline, and then Doc heard about it and went back to save him.
 * No, because Marty was expecting Doc to be waiting at the roof edge in the DeLorean in the first place -- when he hits the edge in anger after looking down it's not because of the height or lack of fire escape but because Doc isn't there. It's set up in the scene where Marty confronts Biff in the jacuzzi, when Biff asks "How did you get past my security downstairs?": Marty bypassed them completely by being dropped off on the roof by Doc. So it wasn't "convenient" that Doc was there -- Marty just had to wait for Doc to fly back before he could escape.
 * Or maybe it was Marty's plan all along.

Marty needed to create an "ideal" 1950s courtship for his parents in order to create an "ideal" 1980s family for himself.
An interesting theory described in this essay.

The time Delorean is responsible for all life on Earth.
At some point, the time machine goes back to when Earth had just formed. Doc (or possibly Biff), unable to breathe, quickly returns to the human era, but leave behind some bacteria, which go on to evolve into everything else.
 * But it can't go back before year zero. There's no "BC/BCE" switch. And it can only do four-digit years anyway.
 * It can only display four-digit years and zeroes on the dashboard. Doesn't mean it can't actually travel that far.
 * Set the display to -4E9. Problem solved.
 * Still suffered from the Ontological Paradox.
 * Wait, you're saying we are all decendants of Biff?!
 * Well, that would explain why humans are bastards.

Twin Pine George was hit by Lone Pine Marty, not by the car that hit Twin Pine Marty.
The Twin Pine version of the same car hit Lone Pine Marty, and Lorraine went all (misnomer-version) Florence Nightingale Effect. Instead of trying to get them together in a different way like Twin Pine Marty, LP Marty sets up a Tricked-Out Time "Loop" (as far as he knows, and for real assuming this is the case, a Tricked Out TimeLoop) the direct way by stealing a car and running it into George. Since he's used to driving, unlike Twin Pine Marty, he expects to be able to stop in time to avoid severely injuring George (whether he manages to compensate for the lack of power brakes and difference in density or if his inability to compensate causes George to be injured enough that Lorraine forgets about Marty for him is up to individual opinion).
 * Note: this theory is dependent on LP Marty being more resistant to Ret-Gone than Twin Pine Marty, since he has to get to the clock tower moment with neither his parents meeting nor his getting too immaterial to drive to pull this off. Fortunately, that is already practically canon, seeing as he or another version of him lives through those exact events trying to protect Twin Pine Marty in Back to The Future II and doesn't appear to fade out...

== George thought he had been visited by a devil, and with proof that Hell exists, was more interested in living a full, enjoyable life with maybe a little pain than living an empty life followed by possibly being toasted on a spit for all eternity thanks to some Deal With the Devil he might have accidentally made. == Dark Father from the planet of the fire god? I know Vulcan's the god of smithing, but that gives him dominion over forge fire and his name's in vulcanization and volcanoes. Punching Biff was keeping himself from committing a sin of omission, and publishing his novel was to make his years on earth more enjoyable since Heaven would become better (less stressful than life) and he might at least get some enjoyment out of life in case that's not where he went.

George figured out that "Calvin Klein" was a time-traveler, and eventually who he actually was
"I guess you guys aren't ready for that, yet. But your kids are gonna love it."

What a strange thing to say - but then, "Calvin" might have just thought he was a musical visionary.

However, "Darth Vader from the planet Vulcan" left an indelible impression on George (judging by the cover art of his book, assuming you can). A SF geek would have avidly watched Star Trek the Original Series during its initial run, and the evening of November 17, 1967 probably didn't pass without poor George yelping at the Vulcan salute. While he did tell Calvin about the event, an angry George would have easily found out that no "Calvin Klein" wrote for Paramount - and more research would have found out that the writer didn't know any Calvin Klein and had never heard of his experience. In fact, research wouldn't have turned up anything about a Calvin Klein who went to his school - just a rising fashion designer who looked nothing like his old friend.

Add to those details all the many little oddities of Calvin's behavior (and his weird "life preserver" outfit when he first arrived), and it would be a very small leap for George to realize that Calvin was from the future - and that he had probably been the "masked alien", as both of them had been very concerned about him getting together with Lorraine. 1977 would have clinched it if George had any doubt. Further pondering about just who would travel through time and care so much about him ending up with a particular girl would lead George to the conclusion that Calvin had to be a descendant of George and Lorraine.

Would he remember the "Dad...uh, daddy-o," slip? Would he dig up his old yearbook and see a picture of his son playing at the dance? Or would he just look at Marty one day and realize that Calvin had been a very immediate descendant of his?

Either way, he'd probably never tell Lorraine, if it could be avoided. That'd just be weird for her.
 * This also explains why he never brought it up. If he actually asked Lorraine on information about Calvin, she'd point him right in the direction of Doc Brown, who would probably tell George not to talk to Marty about it until October 27, 1985, lest it cause a temporal paradox.
 * If I remember correctly, this was actually how the first movie ended in the first draft script (before being replaced by the "something's gotta be done about your kids!" cliffhanger). George sits down at his desk, and on impulse opens up a scrapbook containing a newspaper clipping about the dance, including a picture. He looks at the photo showing "Calvin" playing on stage, and he frowns slightly and says "No... couldn't be..."

Marty actually DESTROYED Goldie Wilson's political career
Goldie managed to go from being a '50s black cleaning boy nobody gave a * beep* to Hill Valley's mayor in 30 years despite not studying when he was young, all by himself. Evidently, this convinced Goldie that he was able to achieve anything he wanted in his life, so he viewed the city hall as just a step before running for state governor and then US president.

Marty's intervention changed it. By travelling to 1955 and telling Goldie he would be mayor, he convinced Goldie he could only be mayor, something his 1955 self definitely saw as an ultimate goal in itself, but his (original) 1985 self wouldn't. The result was a Goldie actually less ambitious, that only aimed to be mayor and sticked in this charge once he achieved it, refusing to run for some higher post and failing to impress his issue into pursuing further political careers. From then on his descendants aimed lower and lower until Goldie's grandson came to feel himself fulfilled despite being just an used car salesman. Thus Marty destroyed both the apparition of an early Obama on the political stage and the emergence of a Black Kennedy-esque dynasty in Washington DC from the 90s onward.
 * No, in 2015, Goldie Wilson III isn't quite old enough to be mayor, he's young like Goldie Wilson was in 1955. Also, Goldie Wilson III isn't a used-car salesman, he hover-converts older cars.
 * Indeed, if you look in the background of the 2015 scenes, there are signs that read "Re-Elect Mayor Goldie Wilson Jr."

From 1 BC to 1000 BC, the DeLorean just subtracts one from the absolute value of the year's number and adds a - in the first digit space.
Doc doesn't know ancient Egyptian, Greek, Babylonian, or any other language in its form from 999 BC or earlier, and he'd look as out of place there as he would in pre-White Man Cometh Aztec culture, and he really only cares about getting cool tech from the reasonably near future and going to the old west, so why bother putting an extra digit in just so you can risk giving a modern, virulent disease that we've adapted to to the direct ancestors of some tenth of humanity?
 * Alternately, the readout could just turn a different color when in the range of BCE years.

Doc and Clara had been traveling with Marty for years before they picked him up at the train crossing.
They already knew everything he did, and were just stopping by to say "Screw The Rules, I've Got An Awesome Time Machine And Know How To Avoid All The Problems I Was Worried About Before, so I'll be picking you up any time you want a ride back... to the future!"
 * They certainly had time to make two kids.

Marty fading in the original film wasn't him being Ret-Gone, it was just him being transferred back to the original timeline.
This also explains the difference between FirstMovie!Marty and SecondMovie!Marty pointed out in the "Changed To Fit The New Timeline" WMG above-- between films, the original and "new timeline" Marties exchanged places, going back to their respective universes.

Those weren't laserdiscs discarded in the alley in 2015; those were DVDs.
Technology would be marching on and some new technology would be replacing them by 2015. I say "would be" because of the WMG below...

The future we live in now is the final result of all the time meddling through the three films.
The future of 2015 that Marty visited was only the result of the time continuum created by the events of the first movie. The results of the third movie are this time continuum we see today, so now, in 2010, there's no hope of hoverboards or flying cars or self-drying jackets or Mr. Fusion devices being developed in the next five years. Nice Job Breaking It, Hero!
 * This means that Marty and Doc are singlehandedly responsible for stopping the advent of hover tech, ubiquitous fax machines, holographic tech, home fusion reactors, flatscreen videophones, scene screens, robotic video waiters, food dehydration/rehydration, and Diana becoming Queen.
 * So Marty killed Diana?
 * This would ultimately result in paradoxes. The DeLorean would never have had a hover conversion, and Doc would have never saved Clara from certain death using the hoverboard. And we wouldn't see the flying train at the end of Part III. Meaning...

The BTTF universe takes place in a parallel universe to our own.
As far as we can tell, the real world and the BTTF world were pretty much identical pop culturally and technologically until at least 1985. Despite the open-endedness of Part III, Doc's train still has a hover conversion, and Jennifer still has that sheet of fax paper from 2015 (albeit a blank one, but bear with me), meaning that BTTF's 2015 still happens much of the same way, hover conversions and all (except that Marty is likely a successful musician).

It's just all in another universe. And we don't get hoverboards like they do.

That's not to say that BTTF's future is completely different from ours. Hover tech notwithstanding, perhaps BTTF's future is aware of things like House, Futurama, Harry Potter, the iPhone, and the Internet--all slightly different, of course, and all of which go unmentioned in the film for obvious reasons. (Heck, you could even argue that the likes of Lady Gaga or La Roux had a more lasting impact on the weird fashion sense of the BTTF future than in ours--just go watch the video for "Bulletproof" and compare the singer's look to some of the people in Part II.)

Speed for time travel is arbitrary based on the traveller's preferences.
For the first test at Twin Pines, Doc sets the speed for 88 mph to show off a bit to Marty ("...yer gonna see some serious shit!"); but the speed remains set at 88 when Marty accidentally jumps back to 1955. However, when Doc first departs for 2015 after dropping Marty off at the end of the film, the time machine definitely does NOT appear to be going 88, meaning Doc must have reset his temporal displacement activation speed (maybe as a safety precaution). That's what the the seemingly useless buttons and doodads on the DeLorean's ceiling are for, as Doc punches them at the end of Part I right before the car flies away. He sure didn't appear to be going 88 when he reentered 1985, and perhaps he reset it to 88 to be a showoff to Marty and Jennifer.

However, the arbitrary speed settings were damaged with the time control microchip when the car was struck by lightning in Part II. The replacement transistors and tubes from 1955 didn't have the speed settings that the microchip had, binding the time machine to the 88 mph needed for time travel, leading to the problem in Part III.

Word of God has it that the car jumped at the end of II because the shock wave from the lightning knocked it into a loop (hence the fiery trails looking like a double six).

Our timeline is the result of another time traveller messing with time.
Even though we already have some rudimentary form of flying car and hoverboard, by 2015 they will not be as advanced and ubiquitous as they were depicted to be in BTTF2. On the other hand, information technology marched much faster than what was shown in BTTF2, and pretty much every electronic device we have can be connected to the internet (which was implied not to exist in BTTF2). This is because another time traveller used his own time machine to change the timeline for his own purposes (just stealing the Delorean for it would result in a time paradox, because it is equipped with technology from a future this action would cause not to exist). More precisely, he went back to 1989 and invented the World Wide Web as Tim Berners-Lee. This caused information technology to progress with unnatural speed, while at the same time slowing down to a crawl every other sector of technology.
 * Corollary to the previous WMG: if a BTTF4 ever gets made, this is what it will be about.

Doc, when he was in 2015 the first time, accidently interfered with the timeline
This caused Marty, Jr. and his sister to get in trouble and jailed. Think about it, Doc is FANATICALLY opposed to any tampering with time even if it does spare suffering. He won't reveal to Marty the nature of his "accident in the future", yet Doc seems DETERMINED to interfere with the future timeline and spare Marty's children any suffering.

Time-travelling has caused many alternate and canon timelines.
Meaning that every fanfic about the series can be true, including "Morty Mc Fly's" cameo in My Immortal.

Before his first trip to 2015, Doc had an undiagnosed cancer.
He says he got his colon and spleen replaced and his blood changed, and that treatment added 30 to 40 years to his life. Now, why would that treatment extend his life so much? The answer is: Doc used to have a colon tumor (more common in old people) that metastatized to his spleen, so the future doctors had to replace those organs to remove the tumoral masses. And they changed his blood to flush out the remaining tumoral cells that circulated through his body.

The 47-year-old Marty was one of the "tranqs" the two cops were talking about.
Assuming "tranqs" are people who are addicted to tranquillizers. Evidence: he activates "lithium mode" on a panel, and lithium being used as a tranquillizer is Truth in Television. The lithium mode he activated was probably lithium being added to their drinking water, which is what psychiatrist Peter Kramer proposed in reality in 2009 to reduce suicide rates.
 * And 47-year-old Marty is pretty depressed.

The timeline would've corrected itself
Biff comes back out of the time machine in 2015 and disappears, but the neighborhood doesn't. Hilldale, for instance, was built in 1985 as a nice neighborhood. In 1985-A, it wouldn't have been built altogether. Furthermore, if Doc was committed, he would never have built the time machine, and never allowed Biff the opportunity. Marty and Doc were destined to succeed at saving the future.
 * If Doc was committed, he would never have built the time machine, and never allowed Biff the opportunity. So he would have never changed the timeline and Doc would have succeeded in building the time machine and allowed Biff to travel to the past, causing Doc to be committed... and so on. This is what the movies call a paradox.
 * That, however, is based on the unspoken assumption that the events would play out exactly the same way in every iteration of the timeline. There's no reason to assume that that would be the case. It could be that the "paradox" would simply be a recursive correcting agent, replaying the events over and over, slightly different each time, until a consistent timeline unfolded.

During the build process, Doc replaced the De Lorean's stock engine
The stock De Lorean takes 10.6 seconds to get from 0 to 60 MPH, and 40 seconds plus a LOT of room to get up to 100. Doc had to include a lead-lined chamber for the plutonium, which must have added significantly to the car's weight. With the stock engine and that much extra bulk, it might not even have been able to reach 88 MPH at all. So he replaced it with a much more powerful engine from a sports car. That explains why the engine noises don't match those from a normal De Lorean.

In the original timeline, Chuck Berry didn't write "Johnny B. Goode"
All Marty says is that was an oldie where he comes from. Perhaps in the original timeline, it was a different artist's song, which Marty then gave Chuck Berry the idea for.
 * We live in the "alternative" time line, i.e. the one in which Chuck Berry "wrote" Johnny B. Goode and George is successful.

Biff decided to enter the auto detailing business because of the 1955 manure crash
We saw how outraged Biff was to be charged $300 to clean and detail his car after crashing into the manure truck in 1955. (Running that through an inflation calculator, it comes to over $2,300 in today's currency - no wonder he was so angry!) Obviously this experience stuck in his mind for a long time (he still remembers it when he travels back from 2015) and the fact that it was so expensive gave Biff the idea that detailing and repairing cars was a good way to make money.
 * Especially because Biff had to have the car cleaned twice in the same week. And the detailer probably charged even more for the second time for Biff's stupidity.
 * Biff probably had to do it himself the second time around (because he was out of money!) and that's what set him down the path to auto detailing.

Griff's gang in 2015 is made up of the grandkids of Biff's 1955 gang
Spike (the girl in Griff's gang) even has a slight physical resemblance to Biff's lackey Skinhead ("get a load of this guy's life preserver").
 * So whose grandkid is Whitey (the Asian guy who says "Unless you've got powah!")?
 * Any of them. Biff's gang (and Biff himself) would have been exactly the right age to serve in The Vietnam War, so it's certainly possible one of them came back with a Vietnamese wife, and if his kid married an Asian-American he could have easily ended up with an Asian looking grandson.
 * Biff was born in March 1937 (just subtract 21 years from the 'Hill Valley Man Wins Big at Races' headline date to calculate this). Therefore, he most likely would have been too old to fight in Vietnam and too young for WWII, but he could have fought in the Korean War.
 * Too old to be in the thick of Vietnam, yes, but the right age to be a ranking officer, yes.
 * But the Korean War went from 1950 to the armistice in 1953, so he'd still be too young.
 * US troops have been stationed in Korea since then. He could've easily served a tour of duty there during peacetime.
 * On that matter, Needles' 1985 gang in Part III are probably made up of the descendants of the Tannen gang members throughout the trilogy. Needles' gang members were played by a member of each Tannen gang: J.J. Cohen, who was Skinhead in Biff's 1955 and 1985-A gang in Part I and II, could be Skinhead's son. Ricky Dean Logan, who was Data in Griff's 2015 gang in Part II, could be Data's father. And Christopher Wynne, who was part of Buford's 1885 gang in Part III, could be his great-great-grandson.

Biff didn't have anything to do with Doc's institutionalization in the "Hell Valley" timeline
Even though Old Biff warns Young Biff about "a kid or crazy, wide-eyed scientist" when he gives him the Timeline-Altering MacGuffin, Doc actually went insane because of his inability to understand why the timeline was deviating so much from what Marty told him. Doc would be the only one in the know about how certain future events are meant to unfold.
 * Wow...that adds alot of Fridge Horror to the Hell Valley timeline. Suddenly Doc's face in the paper says: I've lost every shred of faith. Please. Please, let me die.

When Doc Brown and family visit Marty and Jennifer at the end of Part 3, Doc originally intended to destroy the Delorean and, if neccesary, kill Marty.
He had not foreseen that the train would destroy the flux capicitor, and, knowing Marty's irresponsible nature, predicted Marty would sell the technology. Not able to bring himself to harm Marty, he put it off, knowing he could always go back and do it later. But just in case it didn't come to that, he prepared the 1885 photograph to give to Marty as a gift.
 * ...not to come off as an ass, but I'm pretty sure Doc wouldn't partake in killing his best friend. Now, the idea that Marty would sell the technology is feasible, but I think that Doc knows that, considering how Marty essentially screwed over 1985 in "BTTF Part 2" with the sports almanac, which ended up in Old Biff's hands, Marty wouldn't do anything so stupid. Also, it was asked in Doc's letter from 1885 that when Marty returned back to 1985, he would DESTROY the time machine, and Marty (until it was revealed that Doc DIED in 1885) would honor his friend's wish. But then he had to go back to 1885 to get Doc, and he saw first hand that, thanks to the laws of time travel or some bull-honkey, Doc had his chance at love crushed. So, with Doc decided to, out of grief, reinstate his desire to destroy the time machine, Marty would go along with it for his firend's sake. ...sorry about that rambling there. Bottom line, I don't think Doc would even CONSIDER killing Marty.
 * Why would Doc take Clara and his sons on a mission to murder Marty? Not only would it be emotionally traumatizing, especially for the children--seeing their father kill someone and seeing what the future has in store, Doc knows from experience how easily things can go wrong in the future. That he's taking his wife and sons around in time like it's a vacation is probably evidence that he doesn't care about timelines getting crossed any more, not that he wants to keep them in strict order.

Sonic CD is an alternate timeline of the original Back to The Future
After Marty went into the future and never came back, Doc decided he would rather escape than risk being arrested for life. He creates a spaceship (hey, he built a time machine, so how come he couldn't manage that?) and just so happens to crash into the Past version of Little Planet as he's passing over Mobius. He develops new and fantastic inventions in his laboratory (Wacky Workbench), including a version of the time machine that can operate at lower speeds and without need of a car to do it. A long while after Doc died, Robotnik found Doc's stockpile of time machines in Wacky Workbench (in the present) and decided to use them to take over Little Planet. That secret statue in Wacky Workbench Past that gives you rings? It's Doc's grave.

Needles is Biff's estranged son.
Because come on, every Tannen has a beef with a McFly in every timeline EXCEPT the present one. Needles is just Biff's son from a woman that's no longer with Biff. So he has a different last name and doesn't associate with his Father until he has a son of his own, which he names Griff as a nod to his Father that he accepts him into his family.
 * Potentially Jossed by the game - Edna Strickland (the Vice Principal's busybody sister) yells at Biff's daughter Tiff.
 * Jossed further in Episode 4 by a reference to Mr. Needleman (probably Needles' grandfather), whose mind map suggests him becoming a hooligan.
 * Unless Needles was born out of wedlock and Needleman is his mother's surname.

Jennifer looking different in Part 3 is a side effect of the timeline getting changed around.
While Marty's parents presumably conceived their children at the same or similar time as the previous timeline, Jennifer's parents conceived her at a different point, resulting in the egg being fertilized by a different sperm; the child was still female but looked different.

Doc Brown murdered Lone Pine Marty.
Doc Brown had met Twin Pines Marty and had his letter explaining what would happen. He knew sending the new, spoiled rich-kid Lone Pine Marty back into the past risked messing up the time-stream even more. Therefore, he rigged the car Lone Pine Marty drove in some way, probably to take that Marty to an isolated time period and explode. That second Marty seems to drop off the face of the Earth. There's no indication in the second movie that the Marty seen in the past isn't the original Twin Pines Marty.

Griff is the illegitimate son of Biff's daughter Tiff, who is mentioned in the Telltale game.
This would explain Griff's surname.

When 1955!Doc asked Marty whether something had happened to the Earth's gravitational pull by 1985, he was just being sarcastic.
If some punk kid from thirty years in the future showed up on my doorstep and repeatedly used obnoxious future-slang (e.g., "this is heavy") at me, you'd better believe I'd call him out on it.

Marty is very vain about his teeth.
He refuses to eat anything with sugar and in 1885, members of Mad Dog's gang mentions that he never saw teeth that white and straight that weren't "store bought". Could be Marty had braces when he was younger and now prides himself on having very nice teeth.
 * The refusing to eat sugar could be typical teenager watching his weight, hence why he's drinking Tab in 1985 and asks for Pepsi Free in 1955. And in the late 1800s quality dental care was unheard of.
 * Alternately...

Marty has mild type II diabetes.

 * His family circa 1985-A can't afford to treat it/their insurance won't cover it, so he has to be extremely careful to avoid sugar (which he does) and get lots of regular exercise (considering how much running/jumping/skateboarding, etc, he keeps up on that too) in order to hold off the need for insulin and lower the chances of complications. In 1985-B, his family is simply better off and could afford better medical care for him to begin with, so he's less concerned about his diet (Hence being totally cool with eating a rabbit full of buckshot).

Marty was always sensitive to being called "chicken".

 * He was just never called it in the first film.

2015 is a dystopia.

 * Marty's son is sentenced to fifteen years prison two hours after he's arrested. Why? The justice system moves faster because all the lawyers have been eliminated.
 * And that is evidence of dystopia why...?
 * Lawyers serve a very important role in the legal system. Removing them is pretty much asking for trouble. It was probably a throwaway gag, but it certainly doesn't help given the Cyberpunk/TheFutureIsNoir feel the rest of 2015 has.

The dog statue on alternate 1986's City Hall aren't of Einstein.

 * They're of Copernicus, who he owned briefly in the fifties or thereabouts until

The Bttf Trilogy represents the themes of the first three Metal Gear Solid games.

 * The main theme of the first installment of each trilogy is GENE. The impetus for most of Marty's actions in the first movie is to cause his parents to marry once again, causing him to come into existence from their genes. Likewise, the second movie matches with Metal Gear Solid 2's main theme of MEME. The consequences of information from the future traveling backwards are expanded upon and shown in detail, as Biff uses the Sports Almanac to change his past and eventually 1985. Finally, the third movie fits with Metal Gear Solid 3's main theme of SCENE, since it puts the main characters in a completely different setting than any of the movies offered, and shows how that affects the other characters' outlooks. It's also like the third game in that it shows familiar character-analogues and recurring series events in a historically earlier context.

Certain aspects of time itself is sentient and can affect timelines, and it likes both Martin and Doc.

 * The reason Marty and Doc have time traveled several times without screwing up enough to make any fatal paradoxes is because the sentient parts of time has grown to like them both, and cleans up parts of time to help them out. The fact that the McFly family suddenly becomes awesome, and that Doc eventually found the love of his life, was little gifts from time itself. And the reason they often get into trouble (lightning bolt hitting the De Lorean much? Biff going back in time?) is time wanting to get them into adventures both for its own amusement and them getting amazing memories.

Lorraine has an interesting way of meeting boyfriends...

 * When Lorraine's father (Marty's grandfather) hits Marty with the car, he complains that "another" teenager threw himself in front of the car. This has happened before. Lorraine met all of her previous boyfriends because they were trying to look through at her through her bedroom window and got hit by her father's car. This also helps explain why Lorraine is so taken with Marty: she assumes he was looking at her as she dressed (and apparently she doesn't find this creepy; maybe she even sets it up on purpose) and therefore she assumes that Marty is already attracted to her. In fact...we can further suppose that Lorraine's father suspects that this is the case. Therefore, he tends to hit these boys with his car on purpose (not too hard, mind you) as a way of saying "stop being a peeping tom". He further acts cold towards Marty at the dinner table, because he thinks Marty was peeping.
 * Relatedly, just as Marty is leaving Lorraine happens to say "Marty...that's a nice name", which presumably inspires her to name her son Marty. But she named her first son Dave instead. Why? Because Dave was the name of her first boyfriend who she met due to her father hitting him with the car. Marty was next on the list.

By the end of "Double Visions", everything will be fixed.

 * It's simple logic, really: The promo at the end of "Citizen Brown" shows that . And this had to have happened, because another chunk of the trailer shows that at least Marty (although in non-period accurate clothing) got to 1931,  . Besides, it seems the only two problems left are   Therefore, there's a chance that everything will be corrected by the end of episode 4.

...What? I forgot about episode 5? Well, that one is simple too; you see...

"OUTATIME" will be about Doc and Marty

 * The only two pieces of evidence I can give to support this are two minor things and one Fridge Logic thing. Both minor things come from "It's About Time": First, we have the fact that the only thing preventing Marty to save Doc right away once the DeLorean appears . The second minor thing is that once Marty does get to 1931, the readout takes longer than usual to update itself, where it changes exactly when Officer Danny Parker begins shooting the DeLorean.

Then comes the Fridge Logic thing: in order for Marty and Citizen Brown to fix the timeline in "Double Visions", Brown needs to fix the DeLorean. Using materials at his disposal. ...which means that the repairs he made to the DeLorean will no longer exist once the timeline is repaired, so... you see where I'm going with this?
 * This depends on what those modifications count as. Inanimate objects carried through time are never erased by the alteration of history, only modified. If the modifications count as their own objects, they will survive the prevention of the dystopian 1986, but if they count as being part of the Delorean, they're on the same level as people in a photo, so it's glitch time.
 * And where will our intrepid heroes go? Well, with the time circuits going berzerk, the Delorean wouldn't be confined from the measly range of 0 to 9999 AD. A crossover with Telltale's upcoming Jurassic Park game may occur, courtesy of the actual Jurassic Period (continental drift notwithstanding). And, of course, as hinted in Episode 1, may also be on the cards.
 * Yeah, um... considering how my prediction for "Double Visions" is now Jossed, there's a good chance this is too.

The DeLorean is destroyed/wrecked in every third installment.

 * This is more of a weird idea I had. Concider. "Back to the Future Part III"? The DeLorean was destroyed by the train. "Citizen Brown", the third episode of the Telltale game? The DeLorean A) crashed through a billboard, apparently getting wrecked (pointing your cursor over it at the start of the game has it labeled "Wrecked DeLorean", B) had it's window broken and C) fell from said billboard, and crashed into the ground! Now, maybe it's just me, but I smell something fishy here...
 * Which leads to the obvious corollary, or whatever: The fourth installment sees the restoration of the DeLorean. The obvious examples are:
 * The Animated Series... although it's questionable if it's actually canon...
 * The first episode of the Telltale game
 * And the fourth episode, naturally.

The Doc set an explosive on a timer in his blacksmith shop so no one would learn any new technologies from it.
But, after saving Clara they got back right in time to defuse the bomb and get to work on their time machine.

Clara, building on Doc's explanation of the time travel circuit, was the one who figured out the principles they built the train on.
Doc couldn't have come up with it all on their own.

The components in the hoverboard could be refitted into a crude time machine
It's among their assets.
 * Jossed by the Telltale game, unless they bought a new pink Mattel hoverboard in the future for some reason.

Clara and Doc built a one-shot time machine and used it to go to the future first.
They were able to make only one trip, so they went to 2015 and could get all the parts they needed.

Doc Brown was named after himself.
The Doc's parents, a young couple new to America in the 1880s, changed their last name to Brown after meeting an inspirational and quirky blacksmith, and subsequently named their son Emmett after the blacksmith and encouraged him in scientific pursuits.
 * As stated in the movie, the Browns didn't come to Hill Valley until 1908, well after Marty and even Doc left the old west.

Doc and Marty leaving at the end of "OUTATIME" had a better explanation.
At the end of "OUTATIME", Doc and Marty see  So, what do they do? An actual logical reason isn't given as to why they leave them, but if you think about it, a reason isn't needed.

For you see, just like the audience, THEY don't understand what the {bleep} is going on so they decide to leave as soon as possible. They WILL sort this out... only after they collect themselves to better understand the situtation. And hey, they still have a DeLorean time machine, so...
 * Conversely,
 * This is Fridge Brilliance. Future A!Marty comes back for whatever reason, creating a timeline in which 1986B!Marty apparently decides to eventually come back to that moment because things thereby eventually get screwed up (becoming Future B!Marty/"Evil Twin" Marty), which again screws things up for poor 1986C!Marty, who comes back from the future to settle things as Future C!Marty. Hopefully, 1986!Marty just decides at that point never to revisit that moment in time...

The timeline itself caused the DeLorean to malfunction in Part I, every time it did so, in order to prevent paradoxes.
The first time it malfunctions is shortly after traveling to 1955, when it breaks down in front of what would later become the Lyon Estates housing development, which conveniently has a large sign to block the easy view of the car from a road, and being in an undeveloped area is highly unlikely to be traversed until Marty and Doc can move the car (as it's a weekend, not even the construction or survey crews will be there). Having the starter malfunction prevents Marty from driving the vehicle into 1955 Hill Valley proper, which would garner attention of the wrong sort (which might in turn make it impossible, in some way, for Marty to have access to the vehicle on the night he absolutely needs access to the vehicle - remember, there is plutonium in it!).

The second time it malfunctions is just before the trip back to 1985. The timeline probably causes this because the timer is actually off slightly, and if Marty actually were able to start accelerating when the timer rang he would end up reaching the wire too early, which would cause innumerable problems.

The third time it malfunctions is just after Marty gets back to 1985. The timeline probably causes this to force Marty to go to the mall on foot, minimizing the chance that his earlier self could see him and potentially trigger a paradox. This also answers a Headscratcher question about why Marty only gives himself ten minutes of leeway: the timeline would ensure he couldn't interfere no matter how much time he gives himself. An hour of time, and the timeline might cause there to be a police patrol of the town square at 12:35 that morning. The property damage to the theater front, plus the fact that the DeLorean is not, strictly speaking, Marty's, would be enough for them to detain him at least until they can contact Doc and/or his parents, which would almost certainly be after the Libyan-shooting incident at 1:35. (It would probably take that long just for the paperwork, fingerprinting, etc., and that assumes Marty submits quietly which under the circumstances is not a guarantee.) He gives himself a day or more, and Doc informs him of the bulletproof vest and keeps him hidden until the event actually happens.

So the De Lorean was feeling suicidal at the end of the third film?

In at least one timeline, a daughter (or son) of Marty will marry a son (or daughter) of Doc.
This would explain

In an eventual second season of the game, Marty and Doc will immediately return to 1931.
For two reasons. First, because it would be the cheapest possible cop-out, so it would be the most effective way to troll fans. Second, because Doc saying "The future can wait" means that they are going to the past again.
 * As a corollary, the whole incident with  will never be mentioned again, for the same reason: to troll fans and make them rage.
 * And, of course, as previously established, Marty will never become any of those Marties because there's no way he'll become any of them at this point. Given enough time, they'll fade away, and Marty'll be fine with it because of Doc's 'you're the only real one, and the future is unwritten' philosophy.

Leather jacket future Marty is LP!Marty.
See the headscratchers page for the TP!/LP!marties confusion. LP!Marty will be the new Big Bad of season 2, trying to kill TP!Marty for stealing his place in life. Future Marty #2 is the Marty of the current alternate present as of episode 5.

The Telltale series will eventually end with Marty preventing Doc from inventing the flux capacitor.
The timeline will at some point become such a mess the only way to fix it is stop all of the time travelling from happening in the first place.
 * This will culminate in Marty discovering that the Flux Capacitor was destined to be invented, and Doc invented it a bit later and went through a different time-machine-making process, culminating in a new first experiment not long after his return to the present. The player is given the option of sabotaging it, letting Marty's history repeat itself, or changing Doc's time travel experience for the better.

In the altered timeline that Marty creates through his trip to 1955, Biff has been castrated in the 30 years between then and the "good" future that the Mc Flys have
George McFly could have done this himself to get back at him for attempting to rape Lorraine, or this could have happened any way as a form of karmic justice. Some fans may say that this at least partially explain why Biff is somewhat less "manly" than in the original timeline. However, it definitely explains why Lorraine is (seemingly) okay with seeing her would-be rapist every day and with the fact that he also has keys to her house. She would (possibly) be cool with this because he is not only unable to even attempt this again, but he is also going to be reminded of this (and the fact that he works for his castrator if it was George who did it).

In the first film, Biff was planning on raping Marty.
"You caused three hundred bucks' damage to my car, and I'm gonna take it out of your ass," he says, before throwing Marty to his flunkies. He built up the necessary rage and lust beforehand, with the Ooh La La magazine and booze, while he was planning on attacking Marty at the dance, but Lorraine (who he'd been pursuing for weeks) presented herself as a more preferable target.

Doc burned down his house for the insurance money
Look closely at the newspaper clippings at the start of the first film. We know that Doc is living in his garage and that the land was sold for a large sum of money. Doc claims to have spent the family fortune building the time machine. It's only reasonable to assume that he burned the house down and made an insurance claim. After that money ran out, then he sold the land to a property developer.

The reason why Doc is so cheerful despite everything
He has a goddamn time machine. His second order of business when Marty got back from 1955 was to retroactively cause all his enemies not to exist.

Doc is a fan of Carl Sagan because Carl succeeded where he had failed.
Carl Sagan's deservingness of his fame is something of a point of contention among the scientific community because of how little he has actually contributed to science. However, his immensive skill in getting people interested in science and in providing perspective and aiding in understanding is indisputable. Compare with Doc, whose only admirers were a teenage kid who broke into his garage and that kid's father, who only respected Doc because his son did.

Doc Brown and his family started the race of Time Lords.
It's possible Jules and Verne were affected in some way by the constant time travel, and that frequent exposure to the time stream integrated new powers to the DNA of the Brown clan. As centuries passed, the descendents of Doc Brown made more time machines, eventually integrating space-faring capabilities into the machines. This new breed of humans decide to leave Earth at some point, and found their own, free and open society on a planet called Gallifrey. Down the line, a young man of a race known as Time Lords is visiting 20th century England and gets an idea for a ship design...

The events of the movies are directly responsible for the events of Puella Magi Madoka Magica.
Marty's drag-racing crash in in the original timeline would've killed the father of Hitomi Shizuki when he was still a boy visiting Issei (first generation Japanese-American) relatives in Hill Valley. Hitomi of course becomes the Unwitting Instigator of Doom for Sayaka's (and the show's) Start of Darkness. But the Ripple Effect created when Marty avoided the crash caused the crash 26 years later that injured Sayaka's love interest's (also a budding musician) wrist, and eventually leads to her contract. All their messing around with the space-time continuum had of course put considerable strain on it.

Of course in the previous version where Marty does crash, Marleen becomes a Magical Girl to help her struggling family and deeply depressed father...and eventually a when she's arrested trying to bust out Marty Jr. in Timeline 1a and when Marty gets fired in Timeline 1b. One of the survivors of the crash (Hitomi's would've-been second cousin) became Marty's boss at his office job, initially forgiving and taking pity on him, but nursing a deep grudge that led to Marty's firing.

Before the time machine was invented, Marty was also rather unconfident (he didn't think anyone would like his audition tape), and wouldn't have gotten in that drag race in the first place. But as mentioned above, changing the past the first time around also made him rather more cocky and reckless.

I also wish Doc had never invented that infernal time machine...


 * As you might guess after watching this anime, an implanted the blueprints for the flux capacitor in Doc's head. He thought that with human nature being what he knows, humanity was simply unable to handle the responsibilities of time travel...hence it would prove a great way to.
 * And then of course Doc doesn't get the hint about the problems of time travel after the De Lorean is destroyed, even disregarding his aforementioned paraphrased quote and his previous resolve to destroy it...he builds a whole new time machine!