From what I've read the Directors Cut of The Avengers puts a different perspective on the characters in the spotlight, giving more time to see Cap's adjustment to modern times and I think more into Ruffalo. Want it to come around to cinemas but don't know what is happening with it just yet..
I'm a big Hulk fan, but I'm not confident he could stand another movie alone. I like Ang Lee's take and I flat out love the Ed Norton film, but there's only so much you can watch of "I have all this power, I can't control it... Possible Cure? Oh wait, I'm needed to take on something bad... I must always be alone..." (saying all that though, a planet hulk story could be decent...)
As for Thanos, there's so much laid down that could make him the big bad of this second chunk,
as speculation of the perfect Xanatos Gambit would suggest. Coupled with Fox pretty much needing Galactus as a staple for FF if they can get a reboot together...
The Spiderman thing (so far as I understand it) is that Sony really want to hang on to it for at least 3-4 more movies, and build their own megamash, introducing new villains till they can hit up the Sinister Six.