One More Day/WMG

Mephisto didn't really care about dissolving Spider-Man's marriage
This was never about denying true love, or tormenting souls. This was Mephisto's preemptive strike on Peter Parker and Mary Jane's child. Somehow, the child of Spider-Man would bring bad news to Mephisto, and all Mephisto's bullshit was simply a smokescreen so that Peter would never consider his kid being the target. Mephisto only revealed it after Peter made the deal, and was safe.

One More Day will be retconned as All Just a Dream.
As a Take That to the awfulness of this comic. What's better to negate an Ass Pull than another Ass Pull?
 * I REALLY hope you're right. I honestly prefer the aftermath of Civil War to that of One More Day.

Deadpool and Squirrel Girl are going to team-up, beat up Mephisto and undo One More Day.
Deadpool and Squirrel Girl both know that they're in a comic (Though the latter is contractually obliged to not acknowledge it outside of recaps) and they are both Ensemble Darkhorse fan favorites. It stands to reason that they'd have a grasp of continuity and know when retcons and such happen. Deadpool has been known to have fits of heroism, and might see fit to want to do this one for the fans. Squirrel Girl, being the upbeat, silver-age heroin she is would definitely want to bring Spidey back to his moral roots and undo the Broken Aesop. Unfortunately, she can't acknowledge in-continuity that she knows about it. This is where Deadpool comes in. The way I see it, he's wandering through Central Park ranting about how stupid One More Day was, wondering how he can possibly best Mephisto. TippyToe happens upon him, and drags him towards Squirrel Girl. After an awesome but pointless fight, Deadpool convinces her with TippyToe's help that together they can get Mephisto to undo his massive misdeed. Deadpool and Squirrel Girl then go to whatever hell dimension Mephisto lurks in, kick the crap out of him and force him to undo the events of One More Day.
 * How many times has this been mailed to Marvel? Answer: Not enough, it hasn't happened yet.

The deal will be broken by Peter fathering M.J.'s child out of wedlock.
This will let Quesada keep his precious image of unwed Peter and shatter the whole Aesop about personal responsibility even further. Plus, he'll be able to point at the kid and claim that he gave the fans what they wanted without giving them what they really want. And it'll open up the avenue of letting him make nasty misogynistic claims about poor M.J. being a whore! And won't those issues make such lovely dents in the wall?

OMIT is actually Mephisto editing Peter's memories to ensure he sticks with the deal.
By leading Peter to believe that M.J. was the one driving the deal forward, he gets him to play the victim - "Oh, it was Never My Fault; I'm just a victim of circumstance, just like poor Aunt May!" This is all part of a plan to not only ensure that they stay broken up (and don't try to save their child), but to undo Spider-Man's image as the poster boy for personal responsibility. Eventually, he wants to wear down Peter to a useless, whiny nub who refuses to take responsibility for anything he does, even to the point of giving up being Spider-Man.

Harry Osborn is kept alive so Peter can't end the deal
If somehow the main characters find out what's going on and manage to find a way to end the deal, they can't. Ending the deal would kill Harry Osborn and Spider-Man doesn't kill, especially kill an innocent person. That's the whole reason why Mephisto threw a gratuitous resurrection in there—it's not gratuitous, it's insurance.

Marvel just wanted to retcon Spider-Man and used the marriage deal to distract the fans
By the time O.M.D. came out, huge changes had been done to Spider-Man's setup- his origin was now supernatural, his identity was public, even his powers had been changed. Obviously everything needed to be reset (because Status Quo Is God) and they decided to do a Cosmic Retcon and threw the marriage in as well because they knew the fans would be so mad about it they wouldn't mind the other changes. It's likely that Peter and M.J. will be reunited soon while the changes made will stick. They might have underestimated the fan backlash, though.

Mephisto is aware that this is a comic, and is actively trying to get Spider-Man canceled so he can wipe him out of existence.
Due to his popularity, though, Mephisto knew he'd have to do something drastic to drive away his fans en masse, so he engineered this massive Dethroning Moment of Suck and is working tirelessly to make the series worse and worse to make fans give up on the series out of disgust. When this plot is finally revealed, he will Break the Fourth Wall and openly mock the readers for buying the series anyway, therefore 'justifying' him pulling even nastier crap in the future—after all, he's just 'giving the fans what they want!'
 * And why does he want Spidey out of the picture? Because one by one, any potential allies that his nemesis could have (Ghost Rider if I remember correctly), thus allowing Mephisto to defeat him.

Mesphito is Joe Quesada
Should be self explanatory. See above.
 * Or it's the over way around. Joe Quesada is Mephisto all along, messing with people For the Evulz. This is the first experiment he's done to test his power in both the fictional and real worlds.

Retconning everything after Civil War, as well as the war itself, is actually pretty simple.
It couldn't be any simpler: first, fire a bullet aimed at a certain time in the past of Earth-616 to take out the Mephigeon (without the bullet alone being able to kill him, duh) and then, another to kill friggin' Nitro. Lots of trouble, retconned in a page and a half. Hell yeah!
 * Bullet Points vol. 2! Get on that, Marvel.

Spider-man's Mystic Origin will bite him in the ass, and undo the BND.
Think about it. Spider-man learned that his enemies are totemic, that he is linked to Spider God. That his power was granted, and not an accident. And he even took it even further by eating Morlun's life-source (kinda like in Evangelion). But Mephisto undone it and pissed Spider-God off. Now Spider-God will assemble group of creatures that will assault Spider-man and force him to regain his new powers. This empowering will solve his problem with Web (obviously) and give him his memory back.

Spider-man's Mutant Origin will bite him in the ass, and undo the BND.
Think about it. He was forced to mutate by Queen, and later took it even further by eating Morlun's face, and consuming that rare DNA-mix. But Mephisto undone it, without taking away the initial source of the mutation. Meaning that he will mutate due again. This empowering will solve his problem with Web (obviously) and jolt his memory.

Daughter of Spider-man will undo BND to make sure that she is born, but she will kick her dad in the groin for being stupid and childish.
She has her own Comic Book for God sake! She is a Time lord or whatever...

One More Day is the lead-in to the Grand Finale of Spider-Man.
Marvel wanted to end Spider-man, but had to do something drastic to set off the final story arc. So they decided to create One More Day to do that, and pretty much planned for this whole monstrosity to unfold like it has, more or less. Once Marvel's had their fun, they'll start the final story arc, which will destroy Mephisto forever and leave everybody living Happily Ever After...except the fans, of course.

Mephisto is Trolling the Spidey Fandom.
He has no real sinister plan, no secret agenda, or nothing that he's trying to take care of ahead of time. Simply put: Mephisto loves torment. However, when Aunt May was shot, Mephisto saw through the veils of the fourth wall and percieved the chance to cause the greatest wave of agony and torture in the history of comics. He led Spidey and M.J. into making this poorly thought out choice not to torment them or steal their marriage, but instead to cause agony in the innumerable number of Spider-Man fans worldwide. Every time you cringe at Quesada's poorly executed retcon? Mephisto is enjoying your pain like a light snack. Every time you become enraged over what has been done to one of the most beloved superheroes of your childhood? Mephisto is feeding off your agony like an engorged tick. Thus, this makes Mephisto one of the most evil comic book villains ever.
 * In that case, there's only one thing to do... ONE MORE DAY IS THE BEST SPIDERMAN COMIC EVER! WORSHIP QUESADA AND HIS AWESOME WRITING!
 * But what if...what if Mephisto KNEW you were going to try that tactic, and planned for it in advance by ?
 * Simple, wait for him to get bored. Hey, I can wait until the end of time... Or I can track down one of those neat God-Killing MacGuffins.

That wasn't Mephisto.
Nope. Not Mephisto. Certainly a Reality Warper or Master of Illusion who tried to pretend to be Mephisto, but not Mephisto.

It was Judas Traveller all along! It's funny because he was supposed to have his soul taken by Mephisto in the Clone Saga. What better way to fix a Dork Age than by connecting it to another Dork Age?

Another possibility is that Dr. Doom somehow gained the Power Cosmic; it sounds absurd, but keep in mind the number of heroes that gained their powers, or enhanced their powers, through equally absurd means that should've been fatal. With his new power, he took over Mephisto's powers and used it to imitate Mephisto. Why? Well, because Spidey once had the Captain Universe powers, there were trace effects on his DNA and Doom plans to use any bastard offspring of his to further his own powers.

This is all a brilliant Gambit Roulette by Quesaeda.
No, hear me out before you bring out the torches. Mephisto has immense power to warp reality... in his own dimension. He can't twist the regular world, at least not to that extent. So, my theory is that Peter is actually in Mephisto's dimension, possibly undergoing some form of torture and/ or brainwashing. This could give them a set up for an arc dedicated to busting Spidey out of Mephisto's realm. Such a Plan would require super heroes, previously split by the Civil War, to work together, maybe dealing with the enmity between them. They bust out Spidey in a Big Damn Heroes moment and the world starts to patch itself up. Or is this giving Joe too much credit?
 * I think you are giving him too much credit, but that won't stop his successor from doing that.

This is a different Gambit, but it still fits under this heading. What if Quesaeda somehow got the idea in his head that Spiderman was losing readers, so he concocted an amazing Gambit that would not make itself clear until the very end. He can't exactly KILL Spiderman because, as we all know Death Is Cheap. So, he came up with the idea to do the next most unbelievable thing, retcon his marriage out of existence. Now while everyone is either pissed to the point of borderline murderous rage, or ignoring it, he will simply bide his time, throwing in a few [Chekhov's Gun Chekhov's Guns], and then, when we all but accepted that Peter and Mary Jane will never get back together, somehow find a way to, if not reverse O.M.D. entirely, then a dramatic climax to an arc where they start another permanent relationship again. All of those public statements he says about marriage making him seem too old? All part of an elaborate plan that everyone from Linkara to you are falling for.

OMD will be undone through a Spidey/Ghost Rider team-up.
This is how I see it. Since Johnny Blaze wasn't around during the Civil War arc, he had no way of knowing what Peter did (and the events of World War Hulk had briefly kept him occupied...). But somehow he'll learn what happened, give Peter a savage dressing-down for his lapse in judgment, and then there'll be a quest to restore Peter's memory of everything that happened, followed by a team-up between the two heroes to get Mephisto to restore the Peter/M.J. marriage before destroying the bastard.
 * Also see the Spider-Man page's WMG section.

The Spider-Man from OND/BND is actually Grif
Seriously, who gives CPR to a gunshot wound?!

Mephisto didn't make any deal with Peter. He made a deal with Norman Osborn, who wanted to screw Peter over.
When Aunt May was shot, Norman had a brilliant idea to make Peter's life hell. Norman promised Mephisto his soul, in exchange for which he asked that Mephisto mind-control Doctor Strange (a sorcerer who could easily heal a non-magical wound), Reed Richards (who once built a portal to heaven and demanded that Jack Kirby give a dead Ben Grimm back), and Elixr (whose whole mutant power is healing wounds), and every other useful person in the Marvel Universe into becoming total morons incapable of healing Aunt May. Norman then asked that Mephisto to mind-control Peter into going nuts and thinking that making a deal with the devil was a good idea. Therefore, Norman screwed Peter over by forcing Peter to screw himself over. It also explains why Harry Osborn was resurrected.

Peter is dead now and Ben Reilly is Masquerading as Peter/Sider-Man

 * Think about it, Ben became Spider-Man so Peter could be there for M.J., without the marriage, Peter would be Spider-Man, thus Norman would just kill him and not Ben to bait Peter. So when Norman did kill Peter, he switched costumes, dyed his hair again and said that Ben was dead and that he was Peter to put less shock on people and even though O.M.I.T. states that Mephisto can't harm Peter, Ben is a clone of peter, so he is Peter, and it did not state which Peter he wouldn't harm. This leads us to...

Ben will Pull a Xanatos Gambit on Mephisto to end B.N.D.

 * Ben will go to Mephisto and do a trade, His soul to have the deal never happen and to still have aunt may live without him harming Them or any friends or loved ones. Mephisto will except, only because Ben could be a great bounty hunter for him and the outcomes could be if the deal never happened Ben would not have gone to Mephisto so he still keeps his soul(Ben wins) or Ben becomes a Ghost Rider and will break free of Mephisto (Ben wins).

Peter and M.J. will reconcile eventually, regardless of Joe Quesada, because their love will turn out to be vital to the entire Marvel Universe.

 * As some kind of meta analogy to the fact Peter's marriage to M.J. is an important part of the character, when some kind of shit will hit the whole Marvel fan (read: Crisis Crossover), at some point the only hope would be the help of a divine entity of some sort (Galactus, Beyonder, whatever), but... the god would save humanity only if he deems it worthy. This is the point where It Got Worse: in the judgment of mankind, only a strong love like the former relationship between Peter and M.J. would make the difference, except that - surprise, surprise - it's not there anymore! Then... uh, stuff happens, and at some point either the (results of the) deal with Mephisto are undone for the greater good or Peter and M.J., before the end of the world, spend one more day together until they finally realize they love each other, and then they kiss. Said kiss, being just what's needed in order to save the world, restores the Marvel Universe, reverts the "Crisis" in "Crisis Crossover", and, therefore, is quickly made the single greatest Crowning Moment of Awesome in the entire Marvel multiverse.
 * Just reading this has me smiling. would you be willing to pen a draft for us Peter/M.J. shippers to see?

Joe Quesada ships Black Cat with Spiderman.
Think about it-if he truly hated Mary Jane, wouldn't it make more sense to kill her? The truth is that he would much prefer the pairing between Black Cat and Spiderman than Mary Jane and Spiderman. Hence, he breaks them up so Peter can date and/or bang her without guilt or hassle. The whole Deal with the Devil thing is him being lazy.

Carlie Cooper is a minion of Mephisto
Yeah, you heard me. She's an unbearable Creator's Pet who has other characters pushing her and Pete together because she's supposed to be. Carlie is, in fact, a demon in disguise who's magically manipulating everyone into thinking she and Peter are perfect together. All so she can keep her masters plan intact.
 * Alternatively, Carlie Cooper is Mephisto. He plans to marry Peter as Carlie and trap him in Hell forever.

Proposition of scenario for the inevitable retcon that'll occur once Quesada will FINALLY get kicked out.
Okay, hold on, it's gonna be convoluted. Considering the mess that's this story, it's difficult to make a simple retcon... Shortly before One More Day, Mephisto got defeated by Black Panther. How? The source of Mephisto's power lies within his own realm, so Black Panther cut him from his dimension and easily beat him up (if you think it's ridiculous, remember, it's written by Reginald Hudlin). This defeat made Mephisto mad with anger. To the point that he now wants to collect every single soul of the Marvel heroes. But this defeat also showed him that if he wants to be at his full power in the real world, he must create a bridge from which he'll be able to infinitely tap in his dimension's power source. And he might have found a way.

First, he needs a celibate hero. He could have gone for someone who's single, but he's so enraged by his defeat that for making up for it, he decides to instead corrupt one of the most stable love relationships of the superheroic community (think of it as the little cherry on top of the cake for him). Reed and Sue Richards' marriage being public (and so much more difficult to dismantle in the eyes of the rest of the universe), he decides to erase Peter Parker's marriage. He just waited for an opportunity, and found it when Aunt May got shot. From that point, you can get back to One More Day.

So now, Peter is single. Step 2: bring Carlie Cooper in the picture. Yes, as the above WMG suggests, she is indeed a creation of Mephisto - but she's totally unaware of it. Her personality, her memories, everything about her has been engineered by Mephisto. And if we ever meet her parents or friends from before she met Peter, they're random guys who's memories have been tampered by Mephisto to make them believe they always knew her. For added security, Mephisto gives her the unconscious ability to generate an aura that will make everybody around her think she's awesome and that she's Peter's perfect match (let's call it Creator's Pet effect). So, Peter and Carlie meet each other, and quickly fall for one another; how could it be otherwise, considering Mephisto "built" her to that end?

It's still not enough for Mephisto ; as Carlie is 100% "Mephistelian", if I may say, she can't be used as the bridge Mephisto needs for invading Earth. No, he needs her and Peter's child. A hybrid, part human, part Mephistelian, whose essence is evenly split between Earth and Mephisto's realm. This child would be the bridge Mephisto needs for coming to Earth, at his full power.

Of course, some would start to realize somethings fishy is going on. Doctor Strange would finally notice that somebody tampered with reality, the Ghost Rider would immediately identify Carlie as being linked to Mephisto the first time he'd see her, and people would finally start to listen to Deadpool's insane rantabout Quesada ("Who's Quesada ?") erasing Spider-man's marriage. Too late, however, for preventing the conception of Carlie and Peter's child. Cue gigantic crossover event, major shit happens as the heroes try to oppose Mephisto, massive fights, yadda yadda.

It would end with Carlie recovering from the shock of learning what her true origins are, and proving to be vital in Mephisto's defeat, even at the cost of her own existence. Finally, Peter and M.J. resume their relationship where Mephisto forced them to abandon it, but they'd still honor Carlie's memory, and they'd raise the baby (now purged from Mephisto's essence) with all their love, to prove to Mephisto that despite his powers, he can't oppose true love, between a man and his wife and between parents (even foster parent in the case of M.J.) and their child.

How this mess will be fixed.
Deadpool will be outraged at this atrocity of a retcon. Being that he has No Fourth Wall, he will try and beat up Joe Quesada. Then along comes Mephisto-the true, original Mephisto. He will be pissed that his image was ruined by the false Mephisto, and form an Enemy Mine to get rid of it. This ends up with a Curb Stomp Battle, revealing Joe Quesada himself. After this, Deadpool will give a Reason Why You Suck Speech to him, and why what he did makes no freaking sense. Bonus Take That points if Joe admits "Bah! I don't even care what I've done to Spiderman, I just wanted him to be cool, hip and available!", or even the Anvilicious "I never cared about the fanbase." Whatever the result, it will involve everything about One More Day to be All Just a Dream. And every writer on the block will outright refuse to let Joe Quesada change this. Giving one of the greatest fan revenge moments of all time. And it will be AWESOME.

Joe Quesada will be retconned from history.
REVENGE! If Joe Quesada thinks he can pull a retcon on Spiderman and Mary Jane, we'll pull a retcon on him. The moment we invent a time machine through use of Stable Time Loop.

Prior to writing this story, Joe Quesada had recently sold his own soul to the Devil.
Destroying one of the most meaningful, iconic marriages between two of the most positive yet real human characters in the history of fiction (at least American fiction) while straight up defiling the moral makeup of said characters in the process? Pure propaganda for, depending on the reader, either of two Family Unfriendly Aesops ("making a Deal with the Devil is kind of okay, in fact with a proper enough excuse it's preferable to being Happily Married with child" and "there are no real heroes anymore, as any of the role models you look up to may choose to follow Satan's prerogatives") that are actually part of a bigger one ("human nature is inherently selfish and evil anyway"). Quesada's orders, delivered by Satan, straight from Hell, carried out to perfection upon us. (NOTE: By no means should we rule out any other members of the team as being in on this either. Except maybe Straczynski, because he had a conscience enough to at least say "I want out" for five seconds.)

Mephisto ONLY obtained the power to rewrite reality so blatantly specifically around Spider-Man and Mary Jane because they traded him their marriage.
Think about it. When Mephisto tells Peter and M.J. that what he wants isn't their souls, but is actually their marriage, he practically marks out describing how their marriage is so much greater than owning even both their souls. And in the aftermath of the deal, so much of Peter and M.J.'s history has been rewritten; Mephisto has never demonstrated the power to so grossly alter reality either before or since this point. Thus his power here is frankly seen as a complete Ass Pull. UNLESS... one actually pays attention to his description of what makes claiming their marriage so much more glorious than their souls. Pay close attention to these lines:

""Knowing what's to come there's still more than an even chance I'll get your soul straight-up and that's a far more appetizing prospect. No. In cases like this, I traffic in a far more satisfying currency than souls, Mr. Parker. I deal in the currency of misery, loss, despair, hopelessness, pain, regret, and sadness. They are so sweet to me as the most beautiful music ever composed. I will go anywhere, do anything, for the sound of a soul in pain." "What I want is greater than what one paltry soul can provide me. What I want is the one thing in the universe that is truly greater than the sum of its parts and tastier than any single soul I could devour. I want that which gives you joy, that which sustains you in your moments of greatest despair, the source not of your power, but of your strength, your happiness, your dreams and your passion. I want your love…I want your marriage." "It's because yours is the rarest love of all. Pure, unconditional and made holy in the eyes of He who I hate most. A love like yours comes about but once in a millennia and to take that away from Him…to deny Him…is a victory like none other imaginable." "You will not consciously remember this bargain or this moment, or the life you lived to this point. But there will be a very small part of your soul that will remember, that will know what you lost. And my joy will be in listening to that part of your soul screaming throughout eternity.""

Spider-Man is one of Marvel's quintessential human heroes, in a similar vein to Captain America (comics). An ordinary human who comes to obtain what amounts to peak-human or base-superhuman power, and strives to use this power to do the right thing. Even more so in Spidey's case because away from the mask Peter Parker's life isn't exactly remarkable. He and this beautiful model/actress girl Mary Jane Watson have grown from neighbors to best friends to on-and-off lovers to husband and wife over the course of the comic canon, and their loyalty to each other most of the way through is remarkable. Their relationship, off-and-on, has been iconic for over forty years, and Mary Jane is one of the first things you think of in association with Spider-Man. Theirs is an example of love the world could strive to look up to, and especially in the Marvel-verse after Peter's public unmasking, religious folk could argue it's an example from God Himself. And yes, the God of Abraham, the Most High, the omnipotent Creator that makes every other storied being that ever calls itself a god look like a piss-ant in comparison, does exist in the Marvel Universe, under the name of "the One-Above-All". He even told Peter to be at May's bedside as she dies, implying that the reason every person who should've been able to heal her in seconds suddenly got their hands tied and became useless is because it was simply time to let her go and move on.

Mephisto is basically saying that Peter and M.J.'s relationship and marriage, a model to the world indeed, is actually blessed by the One-Above-All, and that being able to literally take that away from Him is pretty much his grandest victory. It only stands to reason that being able to literally claim something that good and pure for himself would give him the power to so cavalierly alter the lives of the parties involved to the "effect" of this deals results. Notice so many of the changes: Peter's still living in his auntie's house; his best friend Harry was saved by the Goblin serum yet it did nothing to alleviate Norman's insanity or thirst for power; his powers never grew past the point of being stopped by a fat guy falling on him; and precious old May, who he made this deal to save, still the same perpetual old cancer patient, would marry a frickin' Jameson. Mephisto was either trying to change something fundamental about Peter in order to make him a weaker, less-effective hero in every facet of his life so he can't stand in the way of some apocalyptic plan to own everyone, or worse, break him down into a Face Heel Turn so that he becomes Mephisto's version of The Antichrist (as explored by the Dark Fix Fic "Mary Jane Goes To Hell"). And he gained the power for this attempt through taking away Peter's marriage from God.

Of course, this only exposes how colossally stupid Peter and M.J. must be to have accepted the deal after hearing his open gloating, and M.J. earns no sympathy from her tears post-changed-world if she honestly remembers everything. Also, any number of above WMG theories can be tied to this one - including the one about Carlie being Mephisto's personal Sakuya, the one about Quesada being a disguised Mephisto or having sold his soul, the various ways Deadpool, Squirrel Girl, Doctor Strange, Ghost Rider, etc. can take part in the solution that undoes the damage, even the fate-of-the-universe version of the old proposed Clone Saga solution with Judas Traveller and Mephisto's old wager about the nature of humans. Any future Marvel editors or really good fanfic writers reading this and looking for a way to restore the marriage and the characters' integrity with an epic story, you are welcome to all of this!!

Joe Quesada is blackmailing his peers.
That's why they let him get away with this. He's got dirt on the people that could relieve him of his work.

The only reason nobody could save Aunt May was that Mephisto was using his power to prevent her from being saved.
He's Mephisto. Every "deal" he offers is a scam, no exceptions. Why wouldn't he cause the scenario that led to the deal being struck in the first place? A mere gunshot wound being beyond Doctor Strange's magic is blatantly absurd, after all. But suddenly less so if Mephisto were using his own magic to counter Strange's.
 * This only makes it even more likely that everyone's uselessness and the One Above All coming to Peter and telling him to be with May in her passing hours is directly linked. God would know what Mephisto is doing, and that Peter needed to be guided toward the serenity to accept the things he cannot change in order to turn Mephisto's plan on its own head and receive a willing May Reilly Parker into heaven.
 * Fully contemplated in the magnificent Fanfic Breaking the Deal.