Back in the day, I used to play the online text-based MMORPG Urban Dead and really enjoyed Nexus War, a fantasy War-in-Heaven game inspired by its success that a lot of the UD players hopped over to.
In theory, this kind of system would work well on the Fediverse as overheads are relatively low and the instances could represent different "lands".
A quick ponder on this suggests some kind of portal fantasy that gives you access to these "lands" as you would struggle to define a map with instances arriving and, probably, dropping out.
- High fantasy - a bit like Age of Sigma has its "realms" but they are all on a similar technological level. Perhaps have each realm overseen by a particular god or pantheon with a certain ethos, aesthetic or technology (Steampunk dwarves, magical woodlands, etc). You could probably keep everything balanced by having core sets of weapons or skills with added flavour and modest variation in stats (so you could have summoners who would have a set amount of summoning points and could either get one big thing or multiple small things - so an Ent or some Saplings, zombie Hill Troll or horde of zombie rabbits).
- Multiverse - where each instance represents a different "Earth" that has developed in it's own way. Balancing technological systems would be hard as cyberpunk tech would trump Steampunk which would cause problems for Barbarians but Torg managed to do this as hi-tech kit would often break in lower tech worlds.
Something like that. Each instance would work on their particular setting and it could be fun for existing Fediverse instances to come up with a "land" that suited them - db0 would have to pirates, feddit.uk could go Steampunk, SLRPNK could do something... well the theme is pretty clear.