You mentioned online games, so I will throw a random idea that just popped into my mind while reading this xD I have yet to see an online game where you go back and fourth between two worlds. Like, say how in Persona 3 you have your base world with social stats, and then there's Tartarus where you enter a sort of surreal world with monsters, and who knows what else. So the idea might not be original overall, but as an online game, I think it may be.
Maybe say, a dark and light world that affect one another. You can choose to start in either one, maybe each has a different race or something of the sort, but they directly influence each other in some way or another. Maybe if a boss were to die in the light world, a new one will spawn in the dark world after that. Or if a castle or piece of land has been taken in the dark world, then the light world changes in one way or another as well.