"All Good Things..."
Riffing on some of the other ideas (and ST: TNG), in Session 1, the new 5th-level PCs encounter some strange phenomenon. Maybe it's a small, localized dead-magic zone, which acts like a tar pit for magic creatures. Maybe they have to rescue a metallic dragon from it or something.
Session 2, the players' highest-level, most powerful PCs are now living in a world drained of magic. There are dragon graveyards and vast fields covered with the bones of other magical creatures. The elven or fey woods are all dead, wizard towers lie in ruin, etc. The heroes' magic items are inert, or dead in the case of sentient items. (Maybe open with a HAL 9000-esque death scene for an artifact as the last of its sentience drains away. "Daisy...daisy...give me...your answer...do...") Nonhumans are on the brink of extinction, or metamorphosing into humans, or whatever angle you want to take. Maybe the nonhuman PCs are kept alive only by mainlining the last traces of the world's magic like a drug. Anyway, it's bad, and it all seems to have been caused by this planar anomaly spreading throughout the multiverse that sucks the magic out of every world it engulfs. Problem is, as powerful as these PCs are, or were, this world no longer has the magic needed to do anything about the anomaly.
Session 3, PCs from a different campaign, same problem but somewhere in the middle of (1) and (2). Magic is fading but it's not full-on apocalypse yet, only no one is sure what to do.
Each session, move from one world and group of PCs to the next, with adventures focusing on investigating the anomaly and dealing with the various catastrophes it's causing. Eventually, maybe the PCs discover that the anomaly is a rift wherein dwells a Cthulhu Cookie Monster, squatting there like a spider at the center of a web, eating all the magic in the multiverse. All the PCs from each of the worlds have to enter the anomaly together and join forces to kill the CCM. When it's destroyed, all the magic it's consumed radiates back out through the multiverse, restoring all of their worlds. (Alternatively, the CCM explodes, vaporizing the PCs, and while the drained worlds can't be restored, the spread of the anomaly is halted, and the new PCs' world is almost completely spared. But I'm cruel like that.)