First Post Yay! Anyways, i am actually running something similar. The game is going to start up in the fall, and the BBEG is a daemon from the NE plane (forgot the name) that wants to enslave the world as a soul and slave factory. Way in the back when the angelic choirs fought a war to keep something like that from happening, and created a seal to limit Outsider meddling in the material plane. Seal was broken up and distributed across the worlds to make sure no body could get their hands on it. Big surprise, the BBEG has figured out a way around it and now its up to the party to collect the seal shards and reforge the seal so they can re-do the magic that is keeping the hordes at bay.
What i am going to do though is make sure that the whole seal business isn't revealed through at least half way through the campaign (its 3-20 hopefully). before that the party just thinks they are gathering magical artifacts for a well connected Mages guild. The immediate backdrop for the campaign world is that one nation is trying to take over the rest of the world (of course, the nation is daemon controlled, but nobody knows that). so for the first part of the campaign the players are getting their feet under them and traveling to exotic locales to locate artifact pieces for a very high paying client. once its revealed that a daemon prince (relatively low level in the daemonic hierarchies) is trying to take over the world via human patsies and opening gates for his daemonic armies all over the place everything kicks into high-gear as they have to (hopefully) avert the apocalypse. the party should be around 20 when they have to face BBEG, and they may not have to actually fight him.
Sort of similar to what you want to run i suppose. the demon part doesn't come in until late in the game, at least half will probably take place on the material plane facing relatively normal stuff. Once BBEG takes a more active hand in the world it could go more planes-hopping. It is not terribly fleshed out yet. Anyway, hope this helps!
