Every player will be the last of their race in their world. The world may be dying, their peoples killed off, a horrible plague, or any other catastrophe. Perhaps the world is simply old, and the player is a rare immortal, outliving everyone else.
Each player would then be given an opportunity to leave the world, be rescued, what have you.
It's there that I begin to get fuzzy with what happens next. Perhaps they are to be taken to a sort of library of the multiverse to record their knowledge of their world before it is lost. Maybe they are being collected as a sort of 'exhibit'. Perhaps they were preparing to end their life but a being from a higher plane came down to take them back to life away from their dying world.
At this point just trying to brainstorm.
It'd be a 3.5 campaign, and thinking maybe even a Spelljammer campaign.
In fact, considering making it an ECL 20 campaign, but saying they have to each have the Immortal Template] that I wrote up. That'd mean some 12 class/LA levels along with it. Thinking of changing their regeneration to something less mundane. They can choose under which method they can be killed. Instead of acid, perhaps cold, or maybe they don't regenerate from cold iron or blue ice or in areas of dim lighting.
Idea for this concept was actually sparked by viewing of The Islander by Nightwish music video.
Anyways, open to suggestions and discussion.