WMG / It

The Losers Club really did win... on their level of the Tower.
They killed IT for real. They smashed every last egg. Derry was destroyed, and was never rebuilt because there had never been any sane human reason to build a town there in the first place. Any appearance of a thriving Derry and/or Pennywise after 1985 (or the appropriate dates for the TV movie and the 2017 movie) take place on worlds where the Losers lost, or the kids never met, or were never born, et cetera.

It will cause the 2012 apocalypse
It's hibernation lasts roughly 27 years. The last time it woke up was in 1985 and It completely destroyed Derry. Now that King has revealed It's existence to the world, It will be forced to destroy the entire planet when It awakens in 2012 forcing humanity to start over so It can continue to exist in relative peace. At least until someone decides to write another book about It.
  • This is either Jossed (if it ever needed to be in the first place), or after getting his face pushed in two awakenings in a row, Pennywise is sleeping in.

IT is a Youkai

IT was responsible for the Hinamizawa Syndrome.

It being a woman wasn't omitted in the movie.
The "Kiss me fatboy" scene was just Foe Yay.

The true form of the Deadlights is...

An innumerable number of "balloons" glowing with an orange light, floating endlessly. Within each balloon is the soul of one of the children that IT killed, causing them to float.
  • That would actually make some sense.
    • It would also strongly resemble some depictions of Yog-Sothoth.
    • Adding one more thing, if these "balloons" have "strings" attached to them hanging down, would they not resemble a spider's legs?

The Beldam in Coraline is related to It

She just uses a different tactic to get her food.
  • There is a lot to suggest this. It's explicitly stated that IT is female. The final form is a spider. It shape shifts for the purpose of Mind Rape. The other world could be near to the deadlights. Even the setting could be a cryptic clue to the Beldam's true nature. Oregon has doubled for Maine before, this is just Neil Gaiman (a Stephen King fan) doing the same with Derry as he did for Castle Rock. He only gave the setting as Ashland to keep the clue cryptic.
    • And the cat could be a reference to the turtle, the intelligent, perhaps omniscient, animal who lives in I Ts world and counteracts her will. Man, this is getting clearer and clearer.

The Turtle and IT are tied into the Dark Tower universe.

Roland explains that each beam has two guardians on the ends, and one greater demon elemental in the middle. The Turtle is Maturin, and IT is the demon elemental of fear. For that matter, Maturin's death was the result of the ongoing attempts at "breaking" the beam, and Bill and company inadvertently led to further weakening the structure by killing IT.

Stan Uris + Beverly Marsh = Stan Marsh

The creators of South Park got the name for one of the show's main characters from IT. Makes you wonder if a character named Beverly Uris will be introduced on the show.

Patrick Hocksetter is It's Baby-Daddy

Patrick has a great deal in common with It — preying on children, having everyone ignore or downplay his true nature, manipulating Henry to his own end, and reveling in the Losers' fear. When It initially meets him, Patrick is essentially immune to It's shapeshifting illusion. When It kills him, It drains him of his blood and vitreous humor, but It took the form of a leech originally because one once attacked Patrick's cock. It also drags him back to It's lair, where he briefly revives as It devours him, continuing the spider parallels by having It devour It's mate.

In the IT universe, there's no God or Devil.
At some point in the past, someone must have learned about the Turtle and It, and (barely) lived to tell about them. Over time, these stories were retold until they eventually became the God and Lucifer/Satan figures in religion. There's also no Heaven or Hell, and no one goes anywhere after death.

IT is Ungoliant. (Or at least related to her)
Both are Eldritch Abominations in the shape of spiders with insatiable hungers.

IT is IT in A Wrinkle in Time.

Jossed. IT is one of the Associates of the Crimson King.

The man Beverly sat near on the plane was that world's version of Eddie Dean
He's described as tying his ponytail back with a strip of rawhide, which became Eddie's distinctive look in the series. He might even be the same Eddie, since IT happens in '85, before Eddie was drawn to All-World.

Randall Flagg is It's baby daddy.
Making It's offspring Flagg's own analogue to Mordred.

IT will take on the form of slasher icons in the 2017 film.
Such as Freddy Krueger, Jason Voorhees, Michael Myers and so forth.
  • Jossed. A Nightmare on Elm Street got a Shout-Out and IT did appear as something resembling Mama, but the movie is set before Mama came out so it wasn't based on any character's fear of the film itself, the monster was actually based on a painting In-Universe, and IT didn't appear as any popular horror icons.
  • Krueger was considered, but eventually cut out.

It impregnated Itself through the Losers.
The question of how It got pregnant is never answered, but we know of one other funky fertility issue. Seven, actually. Sexless and unable to procreate as It is one of a kind, It sapped the Losers' virility (the male Losers) and fertility (Beverly) to impregnate Itself, effectively stealing their potential children to give to Itself as one more fuck-you. This drain may have also caused Bill and Eddie to start balding early.

It impregnated Itself on a male victim.
It took the form of a female humanoid to rape a male victim before killing him, and the pregnancy stuck through all Its shapeshifting, with the potential children also changing shape in utero and becoming little Its.

It survived because Derry survived.
Its connection to Derry is noted, with the entire town under Its control and the town almost being destroyed when It supposedly dies. Key word: almost. Derry is a reflection of Its soul and serves to help ground Its physical form, so while Its body was seemingly killed, Its true self beyond reality escaped death and remained intact. Derry took heavy damage at the same time Its body died, but parts of it survived and remained habitable, with people electing to remain there and rebuild. By doing that, they essentially restored Its body through Its connection to Derry and It is free to regain Its strength by sleeping and eating more children; if Derry had been fully destroyed and abandoned, It would've been unable to return there and perhaps would've been unable to the physical plane, ever. Instead, It'll keep getting stronger and more entrenched in our world with Its power and influence growing, the surviving Losers none the wiser...
http://tvtropes.org/pmwiki/pmwiki.php/WMG/IT