I would prefer a world that explains why we're here through a very simple and unarguable concept that boils down to "just because" logic like magic or something. The reason why we're here doesn't even have to be our concern, who cares why we're here we are and now we have to deal with this world. Maybe some heroes are spawned in this world and have an entire history to themselves while other heroes are pulled from different timelines for whatever reason. That way you're left with so many different possibilities for characters you don't have to worry about timelines and backstories clashing.