Just powersets, textures (which can then used as a base to reskin models that work for us), entities and talent files. Animations are a very slim possibility.
The amount of slots is written into the game engine, so we have no way of finding that. It's one bytes in hundreds of millions. Models won't work because they use a new version of Alchemy, and sounds are in the 360's XMA format, which nobody on the internet has cracked yet.
So basically we'll only be able to improve on the characters that have already been made, and possibly get new animations.