Frodo doesn't know Legolas' name.

Event Horizon takes place in the Warhammer 40k universe.

The absurdity that the machines in The Matrix were harvesting human bodies for power. An interesting theory is that the machines were using human brains as a cluster of computing power. Cool article.

In the Rock, Sean Connery's character is James Bond. It works well with the Bond timeline, and Connery plays the character exactly as he did Bond. It's fun to see Bond as a secondary character and what it would be like if he was captured.

Snowpiercer is a sequel to Willy Wonka and the Chocolate Factory.

Two in the Star Wars universe. Jar jar was a sith, I love this one and wish they could make it canon. The other is palpatine used padme’s life force to keep Vader alive, I really wish they could’ve explained this better in the movies. And if they went this route, while dark, it is much better for the series in the long run because it really would be a pure evil moment for palpatine.

I also super love the Aladdin is post apocalyptic setting theory.

Among "The Big Lebowski" fans there are some that suggest that Donnie isn't real.
In the whole movie, Walter's is the only person that directly speaks to him. The Dude almost always ignores him or talks to him in a generic/patronizing way.
The theory says that Donnie was probably one of Walter's war buddies in Vietnam and died and what we see in the movie is just Walter's hallucination caused by PTSD. The Dude just plays along to not upset Walter.

I like the theories that the SpongeBob characters are nuclear mutants and represent the seven deadly sins.

In the Michael Bay Transformers movies, the Autobots are actually manipulating humanity into helping them rebuild a military empire, while the Decepticons are actually trying to destroy a much greater threat.

Hans from Frozen was turned evil by the rock trolls.

In the song Fixer Upper, they say 'Get the fiance out of the way and the whole thing will be fixed.' Shortly after, we see Hans be evil for the first time in the film.

In addition, at the beginning of the film, the grand troll notes that it's lucky the magic hit her head and not her heart, because the head can be persuaded. So we have motivation, capability, and a stated intent.

For me, it's the theory that in the original Spider-Man trilogy, Aunt May knows about Peter's secret identity.

I don't know whether the theory has been confirmed or dismissed, but there are quite a few rather obvious hints:

• one scene in the second movie when Spider-Man rescues Aunt May from Doc Ock and he says to her: "We sure showed him." She replies "What do you mean we?" and looks somewhat suspicious and moves her head slightly in an over the shoulder shot, indicating that she may be pondering about Spider-Man's identity after possibly recognizing her nephew's voice. Before that, she was hanging from a building and Spider-Man screams to her to hang on, after which she gives him another uneasy, suspecting look.
• Aunt May's motivational speech later in the same movie in which she states in a very implicative tone that kids like Henry need a figure like Spider-Man to look up to, suggesting that Peter has to continue being the hero he's meant to be. The way she looks at Peter during her speech further indicates that she's subtly encouraging him to keep being Spider-Man. He's about to give up because of all the misfortune he's been having, but she emphasizes her words yet again when she says to "hold on a second longer"; on a rewatch, I noticed that's also when Peter looks up to her as if he realizes that she's speaking directly to him and knows of his struggles. For me, that sentence is the one that convinced me: Peter, the hero, taught Aunt May to hold on when she was at the verge of falling to her death, and now she's repeating his exact words to him.

I like that it's not definitively mentioned in the movies, because it makes for a really interesting debate. I can totally see it being a complete coincidence and that she only cares about Peter and encourages him to be a good person – a hero, as she puts it –, which doesn't have anything to do with being a superhero. So in the end, whether Peter is Spider-Man or not doesn't matter to her. And that in effect means that whether or not she knows shouldn't matter to us.

Disney's Aladdin is actually set in the far distant future.

Deep Space 9: The Prophets are future Bajorans that evolved beyond space & time, which is why they refer to themselves as "of Bajor" and have such a high interest in the fate of its people. The Pah-wraiths are just future evolved asshole Bajorans like Kai Winn & Jaro Essa.

I just re-watched Signs yesterday after what could be over a decade ago. This time I went in with the perspective of the very popular fan-theory that the aliens were demons the entire time. It really enhanced the movie for me and helped me appreciate the Reverend’s story more. The main theme of the movie should be on his story of believing again after the passing of her wife. The illogical actions of what’s considered very advanced aliens detracted from that and the overall movie suffered from this criticism. The theory of them being demons helped to alleviate and even enhance the movie for me and I enjoyed it far more upon re-watch.

Just to be clear , I’m fairly sure Shyamalan’s intention wasn’t for them to be demons as there are a lot of inconsistencies for that to be true. But if you can look past those issues, the movie becomes far more enjoyable with its theme of faith.

Fan theory for reference

In Friends:

Monica is on coke. That's how she got thin and why she cleans so obsessively.

Also:

Phoebe is homeless, saw the others in the group in through the window of central perk one day, and the entire series is all in her head.

I like to imagine The Matrix (1) as a sequel to The Terminator 1&2.

Prometheus as a Christian allegory (spoilers)

I believe Ridley Scott has broadly acknowledged this theory, and, when viewed through that lens, the repeated symbolism throughout the film is unmistakeable.

While I mostly keep watching the show out of some kind of morbid fascination, I've always believed that Rick literally is Morty in some way, or at least that was the originally intended big reveal.

It's just too on brand for the shows sense of humor, and at least in the first season they were really into deconstructing or mocking a lot of classic Sci Fi tropes that while not that well known to the average viewer, any Sci Fi nerd would instantly recognize (in other words, Futurama but with dick jokes), and what fits better into that mold than the Grandfather Paradox?

When I have mentioned this, a few people have said that Rick "refuses to do time travel" or similar, but again what is more on brand for the show than for Rick to just go "Haha fuck you, I always knew how to time travel, but I don't do it because reasons!" or some similar reversal. I mean they spent the first season loudly saying they weren't a serialized story, while dropping breadcrumbs about a grand serialized story.

Apparently the recent anime crossover semi-confirms this theory, but I can't be arsed to watch it, and I don't think it is canon anyways (or if it is, it's from dimension D-414 or something).

Endgame. The Soul Stone manipulated the Avengers into sending Nat and Clint to retrieve it.

The obvious team to go to a dangerous, almost unknown world would be Hulk, the super strong super scientist, and Rocket, the expert space pilot and weapons master. Nat is the acknowledged expert on manipulation and persuasion, much better to talk to the Ancient One. Clint would have been fine going with Thor.

The only reason to send Black Widow and Hawkeye was to force them into making the choice.

That Isayama (creator of Attack on Titan) has been planning for the anime to be a sequel to the manga rather than an adaptation of it.

There's a lot of evidence to support this but the one piece that kind of kicked off the theory is the fact that Attack on Titan copied a lot from a game called Muv-Luv. The ending of Muv-Luv 2 is even nearly identical to the ending of Attack on Titan. In Muv-Luv 3, they introduced multiple timelines and it had a different ending. So if Isayama can copy and ending once he can probably do it twice.

Doctor Who

My own theory is that Rose is always the doomsday device from the 50th Anniversary. The device creates Rose to test the Doctor to see if he is the one. This is how she is able to live after looking into the time vortex and makes Capt. Jack immortal.

Tony Stark knew the tesseract and Pym were in the NJ Shield base because Howard told him about meeting Tony there. It would have happened at some point right before the death of the Starks, as Tony needed to look like his older self. Howard probably told it as an oddity, not realizing it was actually Tony.

Ironman tells Cap something like - I know that I know, but I can't tell you how I know - as if he suddenly put it all together there in 2013. This keeps Cap within the same timeline when he stays.