Eleven films have released so far in what's known as the Marvel Cinematic Universe, with the twelfth (Ant-Man) opening in theaters in just four weeks. This precedent-setting franchise began in 2008 with Iron Man, but between it and its 2010 sequel there was one other film featuring another Avenger, the only MCU film to date that hasn't gotten a sequel greenlit: The Incredible Hulk.
The lack of a Hulk followup is just one of the reasons making Marvel's most uncontrollable Avenger unique. Bruce Banner a.k.a. The Hulk is the only core member of The Avengers who's been recast since his solo debut (Mark Ruffalo replaced Edward Norton) and he's also the only character so far who's movie was distributed by Universal Pictures. Of course, half of the Marvel Studios films to date have been distributed - at least, in part - by Paramount Pictures, but that ended with Iron Man 3 after Disney bought the distribution rights back. Universal however, still holds distribution rights for Hulk.
When talking about the potential of Hulk getting a new solo movie, Mark Ruffalo pointed towards Universal owning the rights being an obstacle, but it's really not. After Ang Lee's Hulk in 2003, Universal opted not to develop a sequel in the contractually allotted time frame and so the actual rights to use the character reverted back to Marvel. Marvel controls all the Hulk-related licenses and Universal still maintains only the distribution rights. According to Forbes (which has uncovered additional specifics on the deal), Univeral retains "the right of first refusal," which means if Marvel decides to make The Incredible Hulk 2 or something like Planet Hulk or World War Hulk, Universal doesn't have to distribute it. They can refuse and then Disney would be able to.
Considering then that Paramount Pictures has been involved in half of Marvel's movies to date and that Marvel is partnering with Sony Pictures on a new Spider-Man - which will be distributed by Sony - the fact that Universal has the option to distribute a Hulk movie should they choose to means relatively little in the long-term planning of the Marvel Cinematic Universe.
The only reason a new standalone Hulk movie is not being made (yet) is because Marvel doesn't want and/or need it in their Phase 3 plans. Instead, Marvel is developing followups to proven intellectual properties (Avengers, Captain America, Guardians of the Galaxy, and Thor), projects that are expected to make a certain minimum at the box office, and investing in new and different properties (Doctor Strange, Black Panther, Captain Marvel, and Inhumans) to add variety to their lineup (and merchandise offerings!). There have been two Hulk films so far and they simply do not hit the high box office numbers as tentpole superhero movies, not even close.
- Hulk (2003) - $245,360,480
- The Incredible Hulk (2008) - $263,427,551
By comparison, Twentieth Century Fox's two Fantastic Four movies both made more than the Hulk movies. And Iron Man, which released just weeks before The Incredible Hulk in 2008, made $585,174,222. There's also the factors of Hulk being a arguably less interesting character from a cinematic standpoint on his own (since he's always big and angry and somewhat uncontrollable) so it'd come down to Ruffalo's Banner dealing with that - which we've seen four times onscreen collectively already. Not to mention, the Hulk portions are more expensive and challenging to animate, so he's better served as a supporting player in ensembles - which Marvel has done successfully thus far.
The other key factor is in the licensing money, since Disney is the top licencor in the world. Hulk merchandise - since it's already a big part of Avengers merchandising - isn't as appealing or varied on its own as something involving new characters either, so there are other factors.
That being said, given that Phase 3 will conclude with the contracts of most of their main heroes also expiring, should key talent not re-sign for additional pictures, Marvel's Phase 4 beginning in 2019-2020 may have to draw upon newer heroes and actors who are still signed, and that could include Mark Ruffalo (who, after Avengers: Age of Ultron, still as at least three pictures left on his contract).
According to what Robert Downey Jr. has been teasing to Mark Ruffalo, we may see Bruce Banner as soon as next year in Captain America: Civil War - in a cameo appearance if anything - but otherwise we can expect to see him back for Avengers: Infinity War and potentially beyond that in Phase 4. Hulk may not be a bankable solo live-action film character now, but he has his own animated series (Hulk and the Agents of S.M.A.S.H.) featuring him with a team of similarly strong and monstrous-looking characters. And there's always potential for Planet Hulk/World War Hulk scenarios in the future.
Ant-Man opens in theaters on July 17 2015, Captain America: Civil War on May 6 2016, Doctor Strange on November 4 2016, Guardians of the Galaxy 2 on May 5 2017, Spider-Man on July 28, 2017, Thor: Ragnarok on November 3 2017, Avengers: Infinity War – Part 1 on May 4 2018, Black Panther on July 6 2018, Captain Marvel on November 2 2018, Avengers: Infinity War – Part 2 on May 3 2019 and Inhumans on July 12, 2019.