The Theory: This film is Stanley Kubrick's way of admitting to faking the moon landings.
Plausibility: This is more of a conspiracy theory than a film theory. As a result, it is completely insane. Most of the cases for this theory are extreme over-analysis of even background props. It's completely ridiculous.
The Theory: The Ghostbusters die after crossing the streams, and the scene where they are celebrated by the city is the last piece of glory they receive before passing on.
Plausibility: Quite plausible, actually. It doesn't make sense that the Ghostbusters could survive after crossing the streams, as Egon explains earlier in the film, and according to this theory, Ghostbusters 2 was the characters in purgatory, repeating the events of the first movie with certain details changed, explaining why no one seems to remember the Stay Puft Marshmallow man attacking New York in the first movie.
The Theory: The Joker didn't actually kill Batman's parents, and Batman only sees the face of his parent's killer in every criminal he sees.
Plausibility: Very high. This is a common part of Batman's psyche in the comic books, and is a nice addition to Batman's character in the movie. It easily fits in, as the detail that the Joker killed Batman's parents is not essential to the plot.
The Theory: There was a third killer, who was Dewey.
Plausibility: This theory seems plausible at first, as Dewey fits some of the descriptions given on the killer early in the film, and had perfect reason to want to kill everyone. However, his appearances later on in the trilogy make this theory highly unlikely.
The Theory: The ending was all imagined by John, while still trapped in captivity.
Plausibility: Considering how the entire mood of the movie changes after John is put into captivity, it is highly possible. Hell, the chamber where he is held confuses the person inside into thinking they are still conscious, when they are actually in a coma.
The Theory: Christine's "curse" is actually a hallucination brought on by her eating disorder.
Plausibility: This is another one I support. There are many references in the film to Christine having a history of being overweight, and on top of that, all of the torture from demons Christine faces are treated like hallucinations from everyone else.