Is there anything like this? Something like Roblox, but rather than using a central server, using a network of decentralised servers with different self-hosted instances (a bit like the fediverse, but for online multiplayer games)

If there’s nothing like this, how difficult would it be to start something like this myself?

edit: Or maybe something like Scratch, where you’re not tied to a specific movement system and can program whatever kind of game you want?