Beavis and Butthead/WMG

Cornholio is a supernatural entity that possesses Beavis and is tasked with protecting humanity from plagues.
In his first appearance, he gives prophecies of "a great plague". Later, in The Movie, his actions derail the arms dealer's bioweapon related plot.
 * And Cornholio is some kind of Eldritch Abomination as well, since he can paradoxically be from a people who are "without bungholes," and yet constantly refer to same. Or, the entity itself has no bunghole, but Beavis does. Thus, he needs TP for his bunghole, yet where he comes from, they have no bunghole. I can't believe I just typed that without humorous intent.

Beavis' last name is Tenma.
Not only does he look similar to Astroboy's father, they're both quite mentally unstable.

Tom Anderson is somehow related to Cotton Hill.
He sounds like Hank, looks like an older version of him, but fought in a foreign war from the mid 20th century. It's possible he's a half brother of Cotton's or possibly a cousin. He could be an Uncle on Hank's mother's side as well.

Either Beavis or Butthead's last name is Sherman
When Principal McVicker is reprimanding Beavis and Butthead in the episode "No Laughing," he clearly says, "Mr. Sherman, your history teacher was unable to teach your class about the Gay Nineties because of you two." It is unclear which of the two boys he was talking to.
 * I think they were just laughing at his class everytime he said "Gay Nineties".
 * McVicker probably meant that Mr. Sherman is the name of the history teacher.
 * Are you sure it wasn't more like "Mr. Sherman, your history teacher, was unable to...", Unless something happened offscreen, he would have no reason to single one or the other out.

Beavis and Butthead is King of the Hill in the future.
Tom Anderson looks and sounds like an older Hank Hill, his wife looks an older Peggy, and when the boys go on Teen Talk, a man there looks like an adult Bobby Hill.
 * This troper once thought the same thing.
 * This theory doesn't explain why the Hill family changed their names. Also, Hank is a lot smarter than Tom Anderson, and he'd never tolerate B&B's antics.
 * Tom Anderson mentions being a Japanese prisoner of war in the episode with the Asian exchange student, much like Hank Hill's father. He is clearly a generation older than Hank.
 * Hank would NEVER say "whackin' off".

Mike Judge salvages characters from B&B to create future shows.
Tom Anderson was used to make King of the Hill, and Van Dreissen was used for The Goode Family. And eventually, there will be Mike Judge's take on the school system with a Principal McVicker Expy.
 * We might see the reverse of this in the Beavis and Butthead revival. Characters from those other series materializing in Beavis and Butthead.
 * Crossover, maybe?
 * I'm excited to see what a show starring a Coach Buzzcut Expy will be like.

The Revival will feature one episode dealing with the 15 years Beavis and Butthead have been off the air.
Assuming this isn't a complete reboot of the series, the first episode of the revival will attempt to acknowledge the missing years. Most likely the duo will lampshade Negative Continuity. "Beavis: Um... Hey Butthead, shouldn't we be old and stuff.

SLAP

Butthead: Shut up bunghole."

The Revival will make jokes about HD TV / analog trun off and how you can't steal cable like how it was done in the past.
Yes Beavis And Butthead had cable do you think they where paying for it? In one episode they even said they where locked out of ordering PPV.

The first episode of the revival will be about Daria moving, leading to her spin-off
Or at least get mentioned.

Beavis and Butt-Head are half-brothers.
In The Movie, according to the drifter who looks like Butt-Head, he scored with "two chicks" and the one who looks like Beavis didn't score. The drifters themselves might be brothers which would explain the Uncanny Family Resemblance.

Beavis and Butt-Head are cousins.
Our heroes actually look a lot alike and share a number of physical mannerisms, most notably their laughter. This troper personally thinks that the claims by the drifter who looks like Butt-Head that the drifter who looked like Beavis didn't score can probably be put down to the same kind of banter their sons share. Like the previous theory notes, their fathers are brothers. Although only Mike Judge would be able to say for sure, it wouldn't surprise this troper if B&B's mothers were also sisters, since it's implied that Butt-Head's mother is a slut just like Beavis's. Certainly it would explain why Beavis or Butt-Head's mother doesn't seem to mind the other kid always hanging out at her house, since he's her nephew.
 * Some of the tie-in books implied they had the same father, making them half-brothers (Jossed by the movie). The same books showed a family tree for the two resembling particularly thick kudzu.
 * That wasn't so much a family tree as an exceptionally inbred family web. Explains a lot, doesn't it?

"Peace, Love, and Understanding" is the final episode chronologically.
Because Mr. Van Dreisen dies by getting run over by a monster truck. That's not Negative Continuity, he really did die. Shortly after, Beavis and Butthead, along with the rest of the stadium, suffocated to death in Sterculius's feces. That's an ironic death if I ever did see one...

Beavis will get to say "fire" after the reboot.
Surely enough time has passed to let him say it again.
 * They might bring back a lot of things that were prevalent in the early episodes, like Billy Bob and Butt-Head's obsession with butts.
 * Mike Judge has confirmed he will be able to say it again.

One of the name mispronunciations in the reboot will be "Bieber and Buffcoat" or something to that effect
You know it will happen.
 * Beavis will not like that one bit.

The reboot will lampshade MTV's Network Decay
At some point, when watching music videos, Beavis will say something to the effect of "Hey, a music video! Do they even have these on this channel any more?". In-universe, he'd just be referring to the channel they're watching, but...

Beavis and Butt-Head have Fetal Alcohol Syndrome
This is why they're mentally slow and have over large heads and deformed facial features. Their mothers weren't just sluts, they were drunken sluts.
 * In the early seasons, a lot of characters had varied designs. But Beavis and Butt-Head kept theirs while the entire series was redesigned to be more "realistic".

Principle McVicker is related to Mr. DeMartino
They're both so over-stressed, that it wouldn't surprise this Troper.

Beavis and Butthead are Obfuscating Stupidity
They've shown some signs of moderate intelligence, which shows that they're not completely stupid.
 * Butt-Head, at least.
 * And both are shown to be fairly competent snarkers while watching TV.
 * They also appear slightly smarter in the early season.
 * In the revival the jokes they make are slightly smarter and Beavis is more of the Deadpan Snarker than he was in the original

Beavis and Butthead are Time Lords.

 * Their couch is their Tardis, and Coach Buzzcut is the Master.

Beavis and Butthead are the owners of Ren and Stimpy

 * Hey, MTV and Nickelodeon are from the same company. It makes sense- who else would be their guardian?

Mr. Manners/Candy will return with another new name and job.
And he'll be beaten up by McVicker (if the latter turned out to have survived the Series Fauxnale, that is).
 * And he ends up jobless and homeless. He's standing at a breadline, along with Gus Baker and Joe Adler.
 * Mike Judge has confirmed McVicker survived his heart attack.

Beavis and Butthead would laugh at the word "rape."

 * The duo are extremely misogynistic and they laugh at anything related to sex, so why wouldn't they laugh at rape? I can just imagine Ms. Dickie lecturing on how to avoid sexual assault and Beavis and Butthead drowning her out in laughter.
 * The episode might be about them screaming "RAPE!".

Beavis's real father is John Redcorn

 * Joseph Gribble has several of the same tendencies as Beavis, including an obsession with fire (several times in the series he's talked about blowing stuff up or burning stuff). Its not out of the question for John to have made love to several women, including Beavis's mother.

Coach Bradley Buzzcut is Native American

 * Buzzcut has a significantly darker complexion than the rest of the characters. His surname might well be a Native American one as well.