The following is a speculative discussion on the future of Hulk in the Marvel Cinematic Universe based on events in the comics. May contain spoilers.
At the end of The Avengers: Age Of Ultron we find Hulk ignoring Black Widow's instructions to rendezvous with the rest of the Avengers and instead, heading to an unknown location.
"One of our tech boys flagged this. Splashed down in the Banda Sea. Could be the Quinjet. But with Stark's stealth tech, we still can't track the damn thing."
- Nick Fury (to Natasha Romanoff)
There's many theories as to where he has gone, many speculative of course. But there's some that are supported by comics. Due to the Marvel Cinematic Universe wanting to be separate from the comics Earth-616 universe, they still draw inspiration from the source material while making some changes like making Tony Stark, not Hank Pym, the creator of Ultron.
After the battle with Ultron, Hulk takes control of the Quinjet showing that Banner is gaining more control over Hulk's actions which could be a precursor his next big moment in the MCU. This plot-point has been deliberately left open-ended so Marvel can decide what they can do with the character after Phase Two.
My theory is that Marvel is attempting to negotiate with Universal Studios to make a Hulk solo-film based on the Planet Hulk comic storyline. Let me explain.
Leading up to Planet Hulk, S.H.I.E.L.D. sends Hulk on a mission to destroy a rogue satellite, but this is a trick to send Hulk away from Earth. The Illuminati (a group comprised of Professor X, Iron Man, Reed Richards and more) use this opportunity to send him outside the solar system. They intended for him to land on a peaceful planet, but the shuttle passes through a wormhole on its way. As Hulk listens to a recording from the Avengers explaining their actions, his shuttle crashes on the planet Sakaar. Weakened from the crash, Hulk is fixed with an obedience disk and taken into slavery. He is forced to fight gladiator battles for the planet's emperor, the Red King.
As to not spoil too much of the comics if you intend to read them, you see that Tony Stark could have planned this. Tony already with the help of Banner created Veronica, a powerful suit capable of dealing with Hulk if he lost control. What if Tony used the Quinjet send him away? The jet is untrackable and according to comic lore, it is capable of space travel. This would be perfect setup for a Planet Hulk film.
My only problem with this is Marvel hasn't developed the Hulk enough to explain the development he needs to go from "Hulk Smash!" to having a decent vocabulary to form sentences in Planet Hulk. This has been an issue the studio has been dealing with for some time and they can't figure out how to put it to screen. If they were to have Hulk meditating and gaining more control of Banner's intelligence on the way to Sakaar, this would help explain why he's changed so much since Age of Ultron for any non-comic readers.
Until we hear of a deal between Marvel and Universal for a Hulk film, we'll just have to guess where Hulk will next appear.