Okay, so, I am planning on being crazy and trying a Planescape 4th E game. So far, I have figured out most stuff, such as factions and races, but I want help with my basic premise, it is quite involved and will take a bit of time to explain, but here is my concept.
Premise: use old cosmology with a twist, use old alignment system.
Idea: Approx. two decades ago, THE general of the grey Waste (yes, HIM, that guy) searched for (and eventually found) something called the well of eternity, something that had not been seen in untold years. The Well of Eternity is on the underside of the spire, at the junction point of all the outer planes. The Well of eternity also is linked to the existence of the multiverse.
While attempting to manipulate the multiverse with the well of eternity, things went horribly wrong. Not knowing exactly what he was doing, the General caused massive (but seemingly unrelated) cataclysms throughout all of reality. Prime material worlds were "blurred" creating an area of intense life (the feywild) and intense death (the Shadowfel) on each prime. Some monsters blinked out of existence, while others were created, and the laws of reality were re-written (resulting in the new 4th ed magic system.) This also resulted in the weave failing in Forgotten Realms, and various other disasters throughout multiple planes. This tampering utterly unmade the General, and thus very few people know what actually happened (yes, I plan to have this come up late in the campaign).
Fast forward 20 years:
After a long and bloody civil war, the Yugoloths finally have a new General of Gehenna. This never-before-seen breed of Yugoloth is ambitious, to say the least. He has decided to end the blood war and stage a grab for power in both Baator and the Abyss. He plans to do this by selling his human-like Semi-Yugoloths (can't remember their name) into the blood war, and having them sew dissention and confusion among the ranks. He also plans to spread lies and misinformation across the planes, and use the belief of other people to subtly influence the blood war to such a point that it will be malleable.
A small secret society of people called the "Order of the Black Lotus" discovered the new General's plan, and they realize he is complemently barmy and that it will never work. However, they also know that it will do SOMETHING and probably de-stabalize the planes for the worse. So they started using cloak-and-dagger techniques to try and counter what the General was doing. Occasionally, they needed to use force. So, they created a fictitious adventuring band, and gave them all the credit for fighting the Yugoloths, in an attempt to divert attention away from themselves.
Unfortunately, stories of this band started spreading like wildfire, aided by what they had (apparently) accomplished, and spread by the lips of every bard and storyteller, pretty soon they were the most popular thing in recent history. With so many people believing in them, they came into existence, as the PCs.
So the PCs pop into existence, knowing their names and who they are, but very little else (they cannot remember anything that has happened to them recently.)