Also, if I can chip in with ideas, I like the idea that of facing some unnatural, faceless entity. It was built to maintain balance, but it evetually grew too powerful and aware. It deliberately shattered the Crucible of Worlds so that it could follow its function and 'restore balance' by consuming all life/data in the Multiverse to reshape it at its whim. And now we have been assembled to reclaim the pieces, destroy the entity, restore the Multiverse to its former state, blah blah blah etc.
So we have to travel around different worlds to find the pieces, but at the same time this entity is traveling around, corrupting and consuming the worlds to harness their power. It gives us a reason to fight enemies characters local to each world, while providing unique twists of 'fighting possessed heroes/villains' and 'escape the wave of corruption' and 'oh crap the entire world is distorting and fighting against us'.
I assume there is no real limitation on worlds, but it should be something in which each Crucible Shard is actually feasible to find (and also something everyone knows, or can learn about quickly).
Sexy. As fuck. <3