At some point in the future the Hulk becomes unmanageable, something happens, some horrible trauma, and afterwards Bruce loses what little grip he had over ‘the other guy’ and things become worse than they ever were.
Since he can’t be killed it’s deiced that the only thing that can be done is to fully sedate him and keep him in an artificial coma for the rest of his life. The plan is cared out and Bruce is kept for the rest of his days sleeping in a secure SHIELD bunker underground in a remote location.
Few people know of his location and the only one who ever visits is Tony. He comes once a year, alone, to pay his respects and give apologies. He was the only one who could get close enough to Bruce to give him the initial dose of sedative and, though he never regrets what he feels was the only chose, he carries the guilt with him for the rest of his life.
Submitted by: tardiscrash