It's risky IMO. I think it could work, but there are alot of challenges. Marvel's Thor (my fav comics character BTW) has never been done well in any other medium aside from comic books. It's a challenging character to do in a serious way. Someone already mentioned Hank Pym above. Captain America on the other hand... absolutely. Especially if the first Avengers film is the resurrection of WW2 era Cap or if Cap gets his own movie beforehand.

Would love to see Avengers done well, but I would hate to see it end up as a modernized version of The League of Extraordinary Gentlemen (or god forbid Misfits of Science

). League X was okay, but it really had alot of problems, not the least of which was character depth and pacing. It'd be a triumph of movie making to have five (or so) main characters, do the setup, explain everybody's motivations, etc. in two hours. More often than not they can't even pull this off very well in the comics themselves.

On a side note, there is enough Marvel movie cannon out there now for a small but impressive Avengers line-up: Iron Man, Hulk, Spider-Man and Wolverine. Also the Beast if you really want to stretch it.