Kudos for bringing that up - “not everyone” / “not enough people” can make it, but we still want to get together / play a game is a *great* opportunity for one-shots, *and* is also a great opportunity for “forever GM” to get to play by mandating that someone else needs to be GM for such one-shots.
On a completely unrelated note, while results may vary by personality type, my experience is, Chaotic “who has something that they’d like to run?” produces more inspiration, more games available in a multi-GM party than Lawful “Thou must be ready to run thine game on the next session”. Also, such Chaotic-minded scheduling is more forgiving of “oh, the PCs got Flight / 20 vials of oil / a clue? That completely invalidates my adventure trapped by a cliff / vs a troll / in the Realms”, giving the would-be GM time to retool their idea, or shelve it and craft another.
(Ok, the Realms aren’t actually invalidated by having a clue, regardless of how clueless many of their characters are portrayed as. By being a Playground Determinator, maybe, but what setting isn’t?)