Old allies coming together, putting grudges aside to reunite for the greater good... It's a classic hook.
Yes, we just got in from seeing The Muppets.
Of course, it generally works as a follow-up to whatever the old allies were allies for in the first place, so in gaming it would generally be a sequel to a game with the same PCs.
But not necessarily - The Muppets owes more than a little of its shape to The Blues Brothers which had a completely made-up backstory. Nostalgia is not required, all you'd need are some listed connections and grudges ready to pick up and run with, as well as a threat big enough to merit the reunion.
Consider also a reunion that only directly affects one of the group. For example, a mentor to younger PCs is contacted by an old friend he hasn't seen in years - Giles in The Dark Age in Buffy S2. It matters to him, but will he involve his protégés?
Or literally, a band. A friend has a Star Wars one-shot based around replaying The Blues Brothers with Max Rebo. Which is pretty close to The Muppets all things considered...