I'm starting to really dig the idea of an Iron Man and Hulk team up movie.
Aside from giving the studio a chance to experiment with an in-between of solo movies and full ensembles, the obvious appeal of seeing more Science Bros, and the epic car chase scene I have in my head (involving Tony in a cool car that houses an emergency armor that he'll need once he gets hit by the death lasers of the AIM caravan they're chasing, and Hulk leaping about, throwing cars and stuff at said caravan... ahem), I think there's some workable business decisions in such a movie.
1. They need a successful Hulk centric movie to save the franchise and potentially lead to a Planet Hulk/WWH. Can't have another average reception like TIH if you want Jade Jaws to have momentum outside of Avengers sequels. Having RDJ co-star could raise the hype by mountains alone. RDJ is only slotted/affordable for A2 and A3 yes, but maybe the studio can bargain with him for one more solo-ish run, especially since Mark Ruffalo could be taking lead and hey, they're best buds.
2. By making it a joint venture, you simultaneously have an effective Iron Man 4 and Hulk 2 wrapped up in one. Give them a common enemy (and I'm thinking a full fledged AIM, which was already established in IM3 but probably far from say... MODOK territory) but their own stakes and it could be a fun romp that advances each character just enough to put them in new places and relationships in time for A3.
....
or maybe the entire idea is terrible and would instantly fall flat on its face if the studio tried it. What do I know? I'm just some guy on the internet.