Dan Vs./WMG

Dan has an English degree
He knows his Shakespeare very well. He has a habit of correcting people's grammar. Plus he doesn't seem to have a job.

Dan is part Neanderthal
Note his sharp teeth, short stature and aversion to vegetables. Neanderthals were almost exclusively carnivores, and their short stature was a result of adaptation to cold climates. This also explains his rather pale skin.

At one point, Dan will try to get revenge on an entire continent.
And of course, it would be for an INCREDIBLY dumb reason.

Dan lives a sad and pathetic life.
He lives in a miserable hotel. He's in love with his best friend, who is married to a bitch. It sucks to be Dan, and so he takes his anger out on everything else.

Elise was the one who smashed in the window of Dan's car in "New Mexico."
After learning of Dan's quests for revenge, she framed New Mexico for messing with Dan's car, knowing he would want to take a trip down there. That way she could just "tag along," and get her revenge without seeming as crazy as Dan.
 * Alternate theory: The person(s) who keeps messing with Dan's car is a random person Dan's been a jerk too. Be it someone he pushed out of the way while walking down the street or someone he cut pass because said person was driving too slow.

Dan is Karkat.
He's short, angry all the time, and always yelling.

Dan isn't gay, or is at least bisexual
Why? Because it'd be a funny subversion where the seemingly gay friend isn't completely in the closet and just hasn't had much luck with women at the moment.
 * The fact that he tries to go on an actual date with an actual woman in "The Barber" seems to confirm this, as well as his short-lived relationship with Hortense in "Burgerphile".

Dan isn't gay, but he thinks that Chris secretly is.
His having Chris say he loves him wasn't wishful thinking. He honestly thought that's the kind of thing Chris would have said and wanted his impression to be as accurate as possible. That's why Dan's own response was awkward and uncomfortable.
 * That's probably why Chris wore a pink speedo in the beach episode.

Ted will be the Big Bad of either the series finale or the season 1 finale.

 * Jossed for the season one part.

Dan is somewhere on the Membrane family tree.
Dib and Professor Membrane inherited his hair color, while Gaz inherited his attitude, temper, and...volume level.

Alternatively, Dan's descendants will marry Chris's descendants and they'll be the ancestors of the Membranes.
All of the Membranes have intelligence not unlike Elise (even Gaz is amazingly adept at technological things) and Gaz and Elise have the same hair color. In addition to that, Gaz also has some of Dan's traits, as noted in the previous guess. Granted, given how unlikely it is that he would get close to anyone other than Chris, it might actually be the descendants of Dan's relatives rather than of Dan himself.
 * Or, the darker version of this theory is that Dan and Elise had a short-lived affair (note how chummy they became in the Washington episode) and the Membranes were descended from the resulting child.
 * Alternatively (and the more in-character than the version above), Eloise never cheats on Chris. She sees that Dan's intellect could be put to potential use in the future, so she takes a sample of his DNA to clone him. However, she also combines it with her own chromesomes, hoping that the resulting test-tube baby will inherit her more down-to-earth nature.

One of Dan's descendants (or relatives' descendants) goes on to found the Angry Marines.
Dan's always angry. ALL THE TIME. One would assume it runs in the family.
 * Considering that Primarchs are somewhat modified clones of the Emperor it would have to be by adoption

Dan has some sort of mental disorder.
In "George Washington," Elise remarks that hanging out with Dan is like hanging out with a big kid. Also, Dan doesn't seem to have a job, or any friends outside of Chris. This seems to hint at a disorder, some form of autism, maybe.
 * Chris has mentioned that Dan has other friends they just can't put up with him like Chris can.

At some point, Elise will invent a machine that will enable Mr. Mumbles to talk...and he'll be voiced by Bill Kopp.
Hey, the guy's already involved with the show, and it's not like he's never played a cartoon cat before.

Dan's last name is Kataiser.
They look similar, and Dan would have reason to be paranoid at everything after nearly dying at least twice. They just managed to rebuild civilisation really quickly by the time he got older.

Ted is Dan's Freudian Excuse.
Ted committed a crime. Dan knows it and testified against him, but Ted got off due to lack of evidence. The crime was was so evil and so horrible that the fact that he got away with it made Dan bitter and disillusioned with the legal system (and eventually all aspects of society), thus making him into the man he is today. This is also why Ted is on his list. Nobody other than Dan even believes that Ted is guilty, thus making their comments about him emotionally low key. If Ted ever actually shows up, he'll be a jarring Knight of Cerebus in a similar vein to The Lich on Adventure Time.

Dan and the Hills are somehow related.
In the episode, Master of Puppets, Bobby did the exact same kind of skyward scream ("Date Niiiight!!!"). Another similarity: Bobby is Camp Straight while Dan is Ambiguously Gay. And you could argue that he has Cotton's temper.

Ted is the one who chopped the tree down in the Vs Washington episode.
He may be stalking Dan, and personally setting up several of the more unexplainable instances that have occured.

Ted is Dan
Not Dan Dan, Impostor Dan. Dan knew that Ted was an identity thief, but decided to keep his enemies close. Of course, this is Dan, so Ted eventually got far enough away that he could come back. Alternatively,

Ted was a girl.
Not just any girl, but Dan's first girlfriend. This little tale takes place before Dan was the Dan we know today, even before he knew Chris. This takes place in highschool. Teddy and Dan were juniors (That's important later) and had been going out for since mid-sophomore year. Just before the end of junior year, Teddy broke up with Dan and started going out with one of Dan's friends. Dan had to watch them all of senior year, and grew very cold. His other friends left him,seeing what he was becoming, and Dan went on alone. Dan met Chris in college, and, when asked about his other friends, said that he had a friend named Ted. Chris never met Teddy or as we know him, Ted. Dan has always had Ted on his revenge list, In fact, Ted was the first name on the list. Dan will eventually take his vengeance on the girl that scorned him, but that episode won't happen for a very long time.
 * Ted's a weird name for a girl, though... Maybe Dan's been the way he is all his life, and Ted was also bitter about life after going through it as a girl named Ted, and they sort of bonded over their mutual loathing of everything. Of course, a relationship like that would be doomed to failure simply because there was just no way for the two of them to avoid pissing each other off, and eventually it reached a breaking point.
 * Actually, Ted is just a tomboyish nickname for Theodaora

Dan is the reincarnation of Napoleon Bonaparte
He is short and angry at the world for no particular reason. All he needs is a Cap'n Crunch hat and he's set.

Dan is part Native American.
The reason he is able to afford his rent and food is because the money gets for being a registered part of the tribe.

Dan and Elise are cousins.
Think about it. Elise's mom said Dan's deviled eggs taste like her mother's and that she's dead. Dan got the recipe from his grandma on her death bed. I don't think either of their families is close knit, so it makes sense.

Dan is/was Skipper Dan.
He's clearly familiar with Shakespeare, and his anger towards the world could easily be caused by one too many soul-crushing days as a tour guide on the Jungle Cruise ride, adrift on a (possibly artificial) lake with nothing but annoying tourists and his own crushed dreams.

Dan is secretly an English teacher.
As mentioned above, Dan does have an English degree, and he does get money from an ATM in "The Bank", so maybe he picked up some sort of job without telling anybody, possibly due to paranoia. He could just be constantly fired due to anger issues, but somehow keeps finding a job, which explains why he'll be broke one minute ("I want my money back!" "That was MY money."), and be able to afford an apartment, cat food, etc.
 * More likely is that he's drawing unemployment, or that he has some other form of income (family money of some kind? I'd pay him to stay away from reunions ...).

Chris will completely snap in a future episode
And it won't be pretty. AT ALL.
 * I can TOTALLY see that: Instead of Dan swearing revenge on something, Chris swears revenge on Dan. Or, Chris snaps and Dan has to spend the episode taking him down so he doesn't nuke the moon or something.

Elise will get A Day in The Limelight
She'll need to get revenge on someone/something as part of a mission, and enlist Dan's help.

Hortense will reappear
The episode will be a non-stop Heartwarming Moment.
 * Oooh ... so close.

Dan's parents will appear and they will be Complete Monsters
They will be a Knight of Cerebus and bring about the darkest episode in the series.
 * They may appear in the same episode with Elise's parents.

Dan is Squee Casil
To begin with, this could easily be the same universe as Johnny the Homicidal Maniac. The wolfman exists, every bizarre conspiracy is real, and rule of funny is a law of nature. Besides, both series take place in the same geographic area. The way I figure it, Squee grew up nasty and prone to lashing out at random people/things partly because of Nny’s influence, and partly because of snapping under the stress of being born a karmic punching bag. He’s short because his parents never fed him properly, and the horrific childhood that scarred that therapist was Squee’s normal life. He shies away from killing, though, because who wants to be too much like as horrifyingly (if amusingly) miserable as Nny? I can’t explain the name change—might be a nickname from college, might be the result of a hilarious anecdote involving the legal system.

Dan will end up with Hortense
Everything he said about Jeremiah Burger at the end will come true.
 * The problem is that Dan, being Dan, wouldn't know what to do with her once he had her.

Everyone that Dan faced will do a Villain Team Up against Dan.
Extra points if Dan doesn't remember and/or they yell DAAAAAAAAAANNNNNNNN!!!!!!!!!
 * The dinosaur did this, so it's possible.
 * So did the ghost of George Washington.
 * Ted will be the leader.

Dan was once a librarian.
Re-watching the episode "Dan Vs. Mall Santa" his resume also lists "Library services".

Dan's name was inspired by Danny Greene.
Danny Greene was an Irish mobster who lived in Cleveland who had a particular skill at explosives and attaching them to cars. With Dan's affection toward his car and one of his first things to use for ventures in revenge being explosives, its an interesting theory on how the creators of the shoe choose the name.

Ted will only appear in one episode each season.
And each would be getting revenge on Dan in various ways.

Dan use to be a psychiatrist.
After hearing so many of of people's problems, he decided he hates everything and everyone.