I think you also have to take into account the contracts and actor schedules if you, say, wanted Cap in IM3. If it is more than a day or two of shooting, that could get messy when you consider Evans is on a 6 film contract, and prob wants to do more films outside of being Cap all the time. So, filming of an IM3 would be taking place when he is pursuing other stuff or relaxing after filming something. Plus, would a pop in appearance in IM3 count in Evans 6 film contract? If Marvel wants 3 Cap films and 3 Avengers films, then they have to be careful of that (same goes for Hems, of course).
Cameos are a great option, but I can see where it could be hairy.