If you actually do mean "MMO" and not traditional "MMORPG", then there are skill-based games that would not necessarily depend on "fully-equipped characters" and could be made to be "Massively Multiplayer Online".

You could have an MMO puzzle game where each federated server can host puzzles that other players have to solve.. and where players can create their own puzzles for others to solve (something in the spirit of "the Castle Doctrine" game, for example, but with fixed gold for everyone).

[–] ICastFist@programming.dev 2 points 1 month ago

A federated mmo could only work if there are no significant attributes/stats to keep track of, or if said attributes are only accepted from bubbles/whitelisted servers. In the end, it'd be something like Opensimulator, which is a FOSS Second Life of sorts.