It occurs to me...it's probably for the best that it would be too difficult to get more characters.
Avengers was so awesome because it built relationships. All the characters have interesting interactions with each other, friendships, rivalries and the like. So we have 6 heroes, Fury, and bad guys. They also pretty much built up Hawkeye and Black Widow as movie characters from scratch, and managed to make them hold there own in awesomeness next to the rest of them. I think they should continue this dynamic. Adding more characters would just distract you from an incredibly good dynamic.
Also, here's a consideration that might switch around the math in terms of buying IP rights. I bet that The Hulk movies, Captain America, Thor, and even Iron Man are all seeing a resurgence in DVD sales, rentals and the like. I think Hollywood might discover that realistically, the owners of something like Spiderman, Daredevil or the like might as well offer to pay for the right to have there characters put in the next Avengers movie. Wanna bet that more people would see a new, say, Daredevil movie if people recognized him from Avengers? Instead of making a new Fantastic Four movie that tanks to keep the rights, perhaps they pay a small sum of money to put Fantastic Four in the next Avengers, and release THEIR movie a month or so later, and watch as legions of Avengers fans flock to their movie, even if it sucks, to eek a little more out of the universe they have come to love? Like bob said, this movie continuity is an experiment, and I suspect that changing dynamics in the business end might be an unintended consequence.
I'm probably not alone on thinking that Black Widow, Hawkeye, or Black Widow and Hawkeye would make a pretty awesome movie. I'm probably pretty close to alone when I say that I think the new Spiderman looks like it could actually be pretty decent. I mean, it kind of looks like it goes back to the web slinger being a technological invention, not a mutant power. That's something, right? Right guy? Guys...?