Groundhog Day

Like Majora's Mask, or Groundhog Day, or The Eleventh Hour. Run a single apocalyptic scenario again and again. Seed the idea way in advance - have a character or entity tell the PCs that there's a group of mages circled up somewhere dedicated to preventing the end of the world.

This circle has been assembled for a single purpose, and have bent their efforts toward it wholly: if an apocalypse happens, rewind time as much as possible. There's a few parts to this. First, a contingent time-stop triggered by an apocalyptic occurence. Then the prime mage, the holder of the contingency, gathers up as many of the circle as fast as they can. The circle, a bunch of casters of 9th level spells, then reverses time as far back as they can. They run the loop as many times as it takes to avert the end of the world. The only people that stay aware are those affected directly by the circle's initial actions. In essence, only the circle knows how many times this might have happened, though their agents may have seen/prevented the end once each, or so.

The PCs happen to be closest to a potential world-ending event. They are alerted to this as the world ends, at which point they feel the reversal, see the Circle focusing their powers, and wind up [an hour/a day/whatever] back. Anyone that would be more appropriate to handle it - Elminsteresque figures - are already committed to actions that support stuff at some stage, or else part of the Circle, or are the next contingent time stop casters, or etc.

Start with a small area. Cut it off from the outside world in some way - extraplanar area, outside of time, antimagic zone, something like that. Create a set cast of characters. Create the chain of events that leads to the End. Then create a chain of events that must be accomplished to get the job done. Ensure that any skipped step prevents overall success. Create events that extend or reduce the time until the End. Create an optimum order of events, and assign each event a time requirement. Time it out so that, if conducted perfectly, it all works. Now release everyone to Groundhog Day their adventure.

A perfect run should look like:

1. Interact with Gatekeeper (6 minutes)

2. Run to the storm cellar, retrieve the Second Seal (2 minutes)

3. Take the Seal to the Blackhouse (4 minutes)

Etc.

So it should total up to a few minutes until the apocalypse. Each step should require a short encounter or check or puzzle. Once a puzzle is solved, they don't have to re-check each time. Thus, it's half meta-puzzle, and half series of encounters.

Elves live a long-ass time; humans a little less so. There are parts of the world where a class system has emerged, where houses are composed of human and elven families both.

The elves provide a kind of institutional memory. They lead the house, play politics, make the long plans. Humans provide support and bodies. An elven house might have a particular human family bonded to them as vassals: a single elven lord living 800 years might be served by twenty generations of the same family, with each human serving as his bodyguard and valet from 25 to 55. Perhaps in his twilight years he forgets which is which, and he's always called them by last name anyway.

The relationship is not unlike favored hunting dogs. Demeaning, perhaps, but there's also a pride to being the first servant of an ancient elven house. The humans' fortunes rise and fall with their longer-lived counterparts.

Throughout all this humans families would likely maintain votes, property, ownership in the family, but it's unlikely they would be the guiding force behind it.

Half-elves in this town would be common, and serve between the elven and human spaces. Awkward social maneuvering might relegate a half-elf to a position much like a human, while successful strategies might put them in power.