This is kind of stolen from a book, but I won't mention the name. That way I don't spoil anything, but some of you may recognise the idea.
It's mixed up with an idea from 40k a bit, too.
um, and prattchett i suppose.
So, your PCs get to 'epic' level. They might just have a lot of experience, or they find an artifact that unlocks epic potential. Either way, they're the only creatures of their power level around.
But there's a reason. That much power shines out like a beacon and attracts dark nasties to their plane, with the urge to feed on their power and lay waste to everything they hold dear in the meantime. There's several ways you can go from here, most of them bleak.
- They voluntarily leave their home plane, taking refuge in the realms of the gods. Same way as many 'ascended' former mortal gods went in the past. Crisis averted. But other mortals could get that powerful again, and they have to find a place among a multitude of powerful beings jealous for space and power. And, if they're conscientious, keep borderline epic characters in line without exerting their full power on the home plane. This cycle can't last forever.
- They could fight the dark nasties on alien ground so as not to risk anyone from the fallout. Only it's dangerous and they might day, but even if they win it'll be slightly harder for the next group of epics.
- They sacrifice their power/get level drained to call off the dogs. But they've only given themselves breathing space because they have nearly epic rivals. What do they do then?
- There's a solution to the problem. and they essentially have to sacrifice their futures to dim the light/lock away their plane. And end the wasteful struggle/strife on the big god home thing. But there's a catch. The PCs have to let their gurard down to do it, and will be helpless against the nasties. But only by working to the very end will they get the salvation project thing to a point where it can bwe completed next time someone gets this far.
You could make a small campaign where they explore the options before finding and carrying out the last one, ending on a bitter-sweet note. To get the best mutiverse, they have to make a sacrifice. But ti's a sacrifice that won't reap rewards for them, but for those who follow in their footsteps. Maybe they start something that will only be finished several generations of epic heroes down the line.
If some of the list is a bit scrappy, I haven't been able to put much effort into formatting and grammar and that. Them's the breaks.