IGN spoke with Mark Ruffalo and Scarlet Johannson, and elicited the following:
Mark Ruffalo: "I think there's a lot of places to go with him [Hulk]."
Scarlett Johansson: "Like where?"
Ruffalo: "Well he's going to space. Yeah, I mean there's some talk of maybe Planet Hulk."
IGN: "Would you like to see that?"
Ruffalo: "I'd be completely interested in entertaining that."So there's "some talk" about doing such a film.
But the thing holding back such a film is that, for a standalone film, the rights are currently in question. Ruffalo said that Universal Pictures still owns the rights to doing standalone Hulk films (the last of which being 2008's "The Incredible Hulk").