General "Thunderbolt" Ross (William Hurt) is returning after first appearing in 2008's The Incredible Hulk for next summer's Captain America: Civil War. In the books, Ross becomes the smarter, more military Red Hulk who also leads the modern Thunderbolts team (one of our Marvel Phase 4 ideas). That's one option Marvel could explore long-term if they want to change things up or see some Hulk vs. Hulk action.
There's also Grey Hulk, the more intelligent Hulk personality of Bruce Banner who appeared frequently in old school Marvel Comics. The special effects teams working on Avengers: Age of Ultron actually designed a grey "berserker" Hulk for use in that film but opted not to use it as to not confuse fans.
And there's She-Hulk, Skaar (the son of Hulk) and other Hulk-like characters and versions from the comics. A bunch of them teamed up and are featured in Hulk and the Agents of S.M.A.S.H. animated series on Disney XD so there's clearly interest in these characters from Marvel's creative teams.
At San Diego Comic-Con 2014, we spoke with Ruffalo about his role in Avengers: Age of Ultron while it was in production and he spoke on the internal conflict of his character, whether Banner can cope with what the Hulk has done.
“Uhhh… if you ask Banner I feel like he would tell you ‘no.’ There’s only so far that he could ever go before he feels like there’s no coming back and we might have crossed that threshold this time (laughs).”
This explains the Avengers 2 ending where Hulk blasts away with the team's Quinjet to hide himself in isolation. As far as Banner knows, he cannot kill himself (he tried) and there is no cure so all he can do is hide. Perhaps it's this reality that plays into how he leaves Earth and joins up with Thor off-world for Ragnarok. Out there in the Nine Realms or other alien worlds could be his salvation... or at least a place he can unleash without endangering innocents.