Whether or not fusion is possible (it seems like it is, just incredibly difficult and meticulous, unless the MIT-ARC project works out).

There is always the eventual option to harvest solar energy in space The primary limiting factor at the moment is obviously the insane price of rocketing stuff into space.

Perhaps even before that solar power could be harvested in the upper atmosphere on massive scale by being tethered to balloons, it might even have the benefit of counter-acting some of the effects of global heating.