Some of Earth's Mightiest Heroes featured in the fast-approaching Avengers: Age of Ultron will be getting solo movies as part of Phase 3 of the Marvel Cinematic Universe (or play a larger multi-film spanning role a la Robert Downey Jr. as Tony Stark) - save, once again, for Mark Ruffalo as Bruce Banner/Hulk. The actor has acknowledged that Hulk solo films are more difficult than other Marvel superhero features in the past, but Ruffalo recently clarified: it's not just the artistic challenges that they present.
Louis Leterrier's Incredible Hulk, which released in theaters about a month after the original Iron Man in 2008, was a Marvel Studios production - but, it was distributed in theaters by Universal Pictures. The latter still has a stake in the Hulk solo movie rights, according to Ruffalo, which is why most of the remaining appearances he's contracted for will likely be in other superhero-led adventures and/or Avengers films (rather than his one stand-alone feature).
Ruffalo provided the following answer to Collider, when asked about how many Marvel films he's still contracted for:
I think it’s four now, but that could always go further. That could include different—I could show up in another character’s movie; I could do Avengers 3 and 4; I could do a Hulk standalone; I could do a combination of those things. That’s four or five, I think.
Banner could feasibly show up in next year's Captain America: Civil War (where the superhero community will find itself split over whether to bow to new government regulations) - though, Ruffalo informed Collider that he doesn't know if that's the plan yet, adding that "Sometimes these things happen at the last minute in this world." It would be appropriate for Banner to at least be referenced (if not show up for a scene or two), in a story that deals with the rights of super-powered individuals in the MCU.
There's also been heavily speculation about something happening to Bruce that would remove Hulk from the equation for upcoming Earth-based Marvel Phase 3 films (like Black Panther) - until Avengers: Infinity War - Part 1 or possibly even Part 2, when he and the other lineup of Avengers are brought back into the mix for the fight with Thanos (Josh Brolin). Then again, Banner was absent for the vast majority of Phase 2 pre-Age of Ultron (his brief Iron Man 3 post-credits scene appearance aside), so really - anything more than that would be a step-up for Ruffalo's Banner, as far as screen-time is concerned.
One frequently-discussed theory is that Banner/Hulk will eventually wind up in space as a means of kicking off a "Planet Hulk" comic storyline-inspired plot in the MCU - a fire that has been put out before, but also one which Ruffalo very recently fanned himself. However, while discussing the idea of any future standalone Hulk films (much less Planet Hulk), Ruffalo told Collider there's a complication:
As far as a Hulk movie, a standalone Hulk movie, Marvel doesn’t really have the rights to that yet. That’s still Universal’s property, so there’s that issue. That’s a big impediment to moving forward with that. Now I don’t think that’s insurmountable, by the way, but I don’t know where it’s going from here for me.
The split-up of rights to the Hulk movie franchise recalls the situation with the film rights to Marvel's Namor comic book property - the latter of which probably won't get a movie adaptation until Marvel Studios has all rights back (including those currently held by Universal). Hulk isn't the biggest priority character for Marvel right now, but it's possible that a deal could be struck between the studio powers that be sometime in the future - allowing for a new solo Hulk movie at last. Hey, Marvel and Sony worked out a deal for Spider-Man, right?
The Avengers: Age of Ultron will be in theaters May 1, 2015; Ant-Man on July 17, 2015; Captain America: Civil War - May 6, 2016; Doctor Strange - November 4, 2016; Guardians of the Galaxy 2 - May 5, 2017; Spider-Man reboot - July 28, 2017; Thor: Ragnarok - November 3, 2017; The Avengers: Infinity War Part 1 - May 4, 2018; Black Panther - July 6, 2018; Captain Marvel - November 2, 2018; The Avengers: Infinity War Part 2 - May 3, 2019; Inhumans - July 12, 2019.