Here’s how I could see it playing out:
• End of the Avengers 2, Banner is sent into Space to investigate some potentially dangerous gamma rays around a satellite. We don’t physically see him leave in the film.
• Planet Hulk, Banner leaves, is in space. Whilst investigating, the Council activate some controls that send Banner’s ship spiralling out of control, and with technology similar to Iron Man’s deep space suit, shoots out across the Galaxy. Banner hulks out whilst this is happening from stress.
• The ship smashes into an alien planet, Sakaar. Hulk smashes out of his ship, and is confronted by guards. They fight him, and Hulk fights back. More guards eventually overwhelm him, and Hulk is taken.
• Banner awakens in a cell, where he meets other gladiators. This is where we’re introduced to the members of the Warbound. Banner is informed he will have to fight gladiator battles for the ‘Red King’. Guards eventually come, provoke Banner. He Hulks out, and is then sent out into the arena. He fights various foes. Basically from this point on the story can follow the basic Planet Hulk story. The pact is formed between him and the other gladiators.
• Here’s where we could change the story. Obviously Marvel can’t use the Silver Surfer because of Fox. However, I feel his role should be replaced with ‘the Other’, who at some point was sent to this planet to find one of the gems in Thanos’ quest for all six infinity gems for the gauntlet. The Red King is in possession of one. Hulk breaks the obedience disk on the Other (who reveals he was never truly under the Red King’s control) but however can now contact Thanos, as he is ‘free’. HE is then the one to offer to bring Hulk back to Earth, under certain conditions. However Hulk, carried away from the battle, declines to leave, whereas Banner would have. The Other then escapes. The Warbound go on the run from the Red King, who actively hunts them after they escape. Continue normal Planet Hulk story.
• This idea isn’t actually my own, it was first mentioned I believe in the Iron Man 3 thread, but I liked it. The Red King is revealed to be the Red Skull, who was transported to Sakaar after his fight with Captain America in CA:TFA. He enslaved the people and created an empire, just like he’d wanted to rule Earth. Of course if Red Skull shows up at some point before this, the Red King will be someone else. They have an Infinity Gem. Hulk defeats the Red King, and the people all wish to appoint him new king. The Other then confronts Hulk again. He demands the gem. They fight, and the other places a reprogramed Obedience Disk on Hulk. The escapes Sakaar and takes Hulk with him.
• In the ending scenes, we see Hulk in a distant galaxy. HE is then confronted by Thanos, who manipulates him into believing that the Council had since him leaving destroyed Sakaar, and that it was the Avengers who had agreed to this plan. Having angered the Hulk Thanos then uses the Obedience Disk to control Hulk.
And this is where it would lead into The Avengers 3. Rather than full on WWH, we’d get elements of it.
• Thanos sends Hulk back to Earth. Hulk, still under Thanos’ influence, attacks the Avengers and the council, causing chaos. He defeats both, destroying the council completely. This draws Thor to Earth, allowing Thanos to invade Asgard, and steal the Gauntlet. He then places the gems into it, and then we have our Infinity Gauntlet story. Hulk will eventually be contained and told that Thanos had manipulated him, Thor learns of the gauntlet being stolen, and then we have Avengers vs. Thanos for the rest of the film.