Since it's apparently being made by the same company that made the Avengers-related movies, I don't think that getting the rights to Cap and Co. will be that difficult, they even mentioned that they were going to work references into Stark into the first episode (getting the actors might be a little trickier though).

The things they're going to have problems with though, are the properties owned by other studios. For example, the Fantastic Four and X-Men are owned by Fox, while Spider-Man's owned by Sony.

So they can effectively get anyone in the tv show that's appeared already in the movies (Agent Coulson for example, and... um... Ghost Rider), but they'd have to forgo mentioning Peter specifically as being Spider-Man or have someone other an Jean Grey getting involved in regards to mentally fixing Jessica.