If there's one thing we know about Marvel Studios, it's that they know how to cover their tracks and create a bigger picture out of tiny references, trivial information and notorious 'Easter eggs'. Since 2008, Marvel has been planning the biggest comic book movie of all time and they've been hyping it up through individual properties; something that's never been done before. For as much as we know about what's going to be happening to the collective members of The Avengers, a question is raised: How is it possible that all these pre and post-Avengers movies are being planned up until 2013, but the one movie that left the biggest cliffhanger yet still doesn't have a planned sequel? I'm talking of course about 'The Incredible Hulk'.
I'm not a betting man, but my guess is that Marvel is going to tie up all loose ends in 2012 and include The Leader as a key player in 'Avengers'. Will he be the sole villain? An accomplice perhaps? Who knows. But you can't just birth a villain at the end of a movie and NOT develop the character; especially such a prominent one that is supposed to be part of an ongoing storyline.
So I believe Marvel is waiting for 'Captain America' to hit theaters next year to reveal the villain(s) for 'Avengers'. They wouldn't be able to reveal the villain because the villain most likely has already been on screen (By that time, anyway.)
As for 'Hulk 2' (if it ever happens), I think Marvel is waiting for 'The Avengers' story to really develop Bruce Banner into a true hero so that by time 'Hulk 2' rolls around, he can finally be part of a movie where he isn't playing the villain in a sense. And I think that after Thunderbolt Ross appears in 'The Avengers', he'll be turned into a greater villain. And as you know, in the comics he eventually becomes the Red Hulk. How visually stunning would it be to see Hulk vs Red Hulk in Real-D 3D!?
(If you're skeptical, you probably don't know anything about Red Hulk. So there's a link to a brief bio on him at the bottom) And T-Bolt most likely agrees to join Tony Stark and Nick Fury in their 'special team' only to get close enough to Banner and get rid of him for good. But since Nick Fury has no plans of destroying Hulk, that won't be happening in 'Avengers'. So maybe the General will get sick of waiting around for others to kill Hulk and he'll do it himself.
http://www.rapsheet.co.uk/rapsheetmain/Character2.asp?UniqueId=1593
So what do you think? While you give me your feedback, I'll be busy writing an editorial on what I think is going on with Hank Pym and Antman.