I think they've covered all the important bases already. Adding new characters for novelty's sake would be a bad move. It requires more time spent on exposition and initial characterisation to get the audience familiar with them, which would be boring, distract people from the actual plot and suck time out of the movie. I mean, it was bad enough with the X-Men and Spiderman films rapidly adding more characters than they could handle. You don't need to do that in a movie that already has a huge ensemble cast.
The whole upside of a crossover-style superhero film like the Avengers is that it goes "You know who all these guys are, you've seen their films, we don't need to explain them to you. Now watch them punch things! Wheee! Punches!"
Anyway. If they were going to add anyone, they'd probably add Luke Cage. He can be easily tied into Captain America's background, and it's good to have more black guys on the team. Otherwise, the safest bet would be Hank and Janet Pym. Their backgrounds are easy enough to explain (Pym particles make you big/small) and their characters can easily be summarised as "disintegrating married couple," which is a nice, short arc to fit into an ensemble movie.