Originally Posted by ThePowerCosmic
I can easily see Hulk losing control in the fight against Thanos and going primal, while the world and The Avengers watch in shock. Hulk might even have to be subdued by his teammates. After all that talk about finally having the beast under control, Banner wouldn't be trusted and Hulk would be sent away where he couldn't do anymore damage. Maybe he could even be sent to a prison established by the Guardians, but his ship gets knocked off course and he ends up on an alien planet. There, he learns to truly control and use his power, becoming a much more experienced fighter and leader as well. I could see him using leadership skills he's learned from being around Cap.
I like this theory, sans the uber leadership part. Banner just isn't the leader type, and while Hulk can lead by sheer of example of his brute power, it wouldn't last.
That aside, it makes a lot of sense for the Hulk to go blind with rage in the film and start attacking everything, which results in the Council deeming him too dangerous to the planet and shooting him out into space.