The specifics of how Age Of Ultron will end are, obviously, unknown. One popular theory is that the fallout of Hulk losing control and causing Tony Stark to get out his Hulkbuster armour, will see Bruce Banner getting shot into space, kicking off a big screen approximation of the Planet Hulk line. In the comics it's the Illuminati who does this, but as that secret group includes Professor X and Mister Fantastic (both owned by Fox) in its numbers it could easily be some part of the reforming S.H.I.E.L.D. The post-credits scene could see him getting rocketed off-world or actually show him landing on a new planet. The big thing working against this is, quite simply, there isn't a Planet Hulk movie for this to set up. The introduction of Spider-Man showed that the Phase 3 plan isn't set in stone, but it'd still be a surprise for Marvel to foreshadow Phase 4 already. However, that doesn't rule out appropriating the plot point into what's already there; Hulk being in space could see him come up against Groot in Guardians Of The Galaxy 2 or Planet Hulk being somewhat worked into Infinity War (although that really steps on the toes of other great comic book arcs). The latter seems pretty likely. Likelihood: 7/10