The thing with all these comic book movies is that they have decades of material to draw from. You can make an X-Men movie with any combination of mutants and it would probably work. The Avengers can be treated in much the same way.
I would love to see more Black Widow. I had no real opinion of her in Iron Man 2, but I thought she was a great addition to The Avengers.
Frankly, I'd love to see another female superhero be added to the mix. Let's throw someone like Ms. Marvel in there.