Well, you need a Banner Hulk to do Planet Hulk. I came up with a theory on how to do it, I'll edit it in later when I find it, but the short of it was Ultron takes out Banner somehow, Hulk RAGES, stomps Ultron, turns on the Avengers, SHIELD launches him into space.
Banner is trapped in the HULK's mind like a prison, forcing him to face the beast within.
*that's when he gets launched into his own movie*
At some point he learns to be a fully conscious HULK, and Planet HULK happens. He somehow gets off the planet and by luck (crash lands on Nowhere?) meets up with the Guardians who then bring him back to Earth just in time for Avengers 3.
Now when Hulk returns, Banner is in full control of the beast and is fully able to transform back into Banner at will.