Here's one of my personal favorite rumors that was going around about the Marvel continuity: Avengers 2 would end with a fearful earth launching the Incredible Hulk into space. This would lead into a standalone Planet Hulk movie, in which the Hulk landed onto a random alien planet and proceeded to conquer it, only to return in Avengers 3 and take his revenge. That's...probably not gonna happen. But here's another rumor that very well could. *MAJOR SPOILERS IF TRUE*
To start off, here's a little backstory: In the comics Ultron is an A.I. created by Hank Pyme aka Ant Man, which turns evil and proceeds to kill a bunch of people. It's been confirmed that Ant Man will have nothing to do with Ultron in the film, so this begs the question: Who will create the villain? The most likely answer seems to be Tony Stark. After all, it'd be a great character arc for him to come out of superhero retirement to defeat something that he himself created.
Now, here's where the rumor comes in, and easy to believe because it's actually pretty easy to come up with on your own. Who is the AI companion from the Iron Man films? Jarvis. The rumor is that somehow, Jarvis will be the A.I. that turns evil, becoming Ultron and probably killing off a few important characters. Is this true? It's easy to believe. Heck, whoever came up with the rumor could be right by accident, if this isn't something from the direct source.
Avengers: Age of Ultron will be released on May 2015.