Well, sure, why not? I can only imagine that Joss Whedon is planning ahead for multiple films in the Avengers franchise; I'd bet good money on a three-picture story arc to be set up in the first film and some shadowy enemy to appear in the background for the sequels. But who? I think Dr. Doom is tied up with Fox's Fantastic Four movies, but I guess there's no shortage of iconic Marvel villains that they could bring out. The Skrulls? A.I.M.? M.O.D.O.K.?
"They set up several pictures over a couple of years and possibly there will be a Hulk movie. There'll probably be a couple more Avengers too, which would be fun."
I'm also intrigued by the talk of a new Hulk movie. The first Hulk was widely perceived to be a disappointment, but the second movie only made a couple million dollars more than it did. I'd be all in favor of a smaller, lower-budget Hulk movie that stuck closer to the ethos of the old TV show than some huge blockbuster, but that doesn't exactly seem to be Marvel's M.O. lately.