DISCLAIMER: This is just the inkling of an idea for a world, don't expect much to be here yet.
I think I want this world to actually be a pair of worlds that are linked in some way. Maybe everyone in one world has a corresponding person in the other world that they are linked to? Maybe one world in human, the other is some sort of spirit world, or world of beasts of some sort, which people are linked to? Maybe the two worlds are just opposites in some fundamental way. Maybe some key point in history when in two different directions, creating two totally different results?
Either way, I think that at least one of the worlds in unaware of whatever this link is, and there will probably be people exploiting the link in some way.