The core of the cast are the big-name heroes who they felt could support their own solo films (Iron Man, Thor, Captain America, Hulk; plus, with the first three, it really wouldn't be an Avengers film without them). They introduced Black Widow in Iron Man 2. There's not really any opportunity to introduce others prior to the film being made - if there was a Hulk sequel, they could have brought in She-Hulk.
when Jeremy Renner first mentioned having talked to Marvel, back last year in Empire mag, he said Hawkeye was appearing in Captain America. if that's true, it would a) probably be in the modern book-ends and b) not be too different to them using BW in IM2 and re-using her in Avengers.
i'm slightly surprised Rhodey/WM ain't in Avengers though...