MarkB
Legend
I've been planning a prophecy-based game for a long time, about a world where everybody knows that Fate controls everything. Prophecies are very clear and always happen exactly as written. Until the PC's come along. It's an idea inspired by Sparhawk (a.k.a. "Anakha") from the Elenium; the PC's, for whatever reason, are simply invisible to the Fates. They are a wildcard in the prophecies: the only way one can change is if one of the PC's (or a few other such individuals in the world) get involved somehow.
I've thought about doing something similar - having the PCs plane-shifted into such a world, the first outsiders ever to enter it and the only ones whose fate cannot be seen.
But I was going to add the twist that, through Chaos Theory effects, once their presence starts unravelling a few prophecies, it has a knock-on effect that, pretty soon, spreads to spoil all prophecies - and since this world used divination in almost every aspect of peoples' lives, from the choosing of alliances between nations down to minor daily predictions that tell people whether to bring an umbrella to work that day, the world's whole society starts spinning out of control, and the PCs are pretty much the most unpopular people on the planet.