I don't get why everybody wants Ultron for the sequel. Thor can just throw Mjinor at him and it'd end, or Iron Man fire his repulsors. I think Red Skull and HYDRA would be interesting, or Kang the Conquerer. I have a theory that Phase II will consist of a search for the other Infinity Gems, hence why the heroes go their seperate ways. It could turn out that Kang (or whoever the villain is) is pulling the strings behind their own movies's sequels and sending them off the trail of the Gems. Pure speculation but it'd be cool.
Also, we've heard that Manadarin won't be the villain of IM3, as Shane Black doesn't seem too keen on him. Maybe that's because he's the villain in Avengers 2? That wouldn't be enough for The Avengers to re-assemble and add members (or replace members, nugde nudge wink wink) so maybe Mandarin, Fin Fang Foom, Red Skull and HYDRA!? It'd be truly epic if that happened. Another thing, I think Hulk might leave in this one. I just know that he'll find a cure to his disease and leave, I won't be suprised if it happens. I've got a feeling that Ant-Man, Wasp and Black Panther will come to the team. Who else will leave? Once SLJ leaves Maria Hill will no doubt join the group and Black Widow will probably betray the team like in the comics.
Pure speculation. Or is it...?