While Mark Ruffalo recently stated a Planet Hulk movie wouldn't work because the comic book storyline didn't feature Banner, new rumor and speculation has hit the net regarding the fate of the Incredible Hulk in The Avengers 2.
HitFix, for some reason, is allowed to reveal the Hulk's fate in The Avengers 2. It's said at the end of The Avengers: Age Of Ultron, it ends with the Hulk onboard a Quinjet off his way into space, with no way to turn around as the craft is preprogrammed and can't be stopped.
My speculation: It's possible Ultron might be onboard the ship as The Avengers attempt to fly Ultron out into space to rid the Earth of its presence.
The report then goes on to speculate that the Hulk will meet up with the Guardians of the Galaxy out in space, which eventually leads them all back to Earth.
This is rather interesting as it does borrow heavily from elements of the comics.
What is also interesting, if Ultron is onboard the Quinjet, this could possibly play into the Marvel Cosmic event known as Annihilation: Conquest, which saw Lord Ultron control the techno-organic Phalanx as they took over the Kree. Interestingly enough, this is the comic book event the current (and movie team) of Guardians of the Galaxy sprung from.
More than likely, Marvel Studios will not be directly following the comics (as they didn't with the Guardians of the Galaxy movie, but were inspired from Abnett and Lanning's books), and we see that is true with the addition of the Hulk.
Obviously, a lot of this is speculation, but we could possibly see elements of Planet Hulk, such as the gladiator fights, and maybe the Guardians are present to help free the Hulk (Guardians even appeared as Easter Eggs in the Planet Hulk animated movie - see below).
It's also speculated the Hulk teaming up with the Guardians will involve Thanos and possibly The Avengers 3 or further into Marvel Studios plans.
It might even be possible that when the Guardians and Hulk arrive back on Earth, they battle The Avengers, which would be a take on World War Hulk.