The plan for this year is to add more game apps to Freedombone. The intention isn't to turn the system into a gaming platform, but running a game server is one way that people can learn about self-hosting in a more accessible and fun way. Unsurprisingly there aren't all that many Free Software game servers out there. But there are a few, and some of them are packaged for Debian. Recently I've added a couple more as Freedombone apps. Neither of them are playable on mobile.
Crossfire is a rogue-like role playing game almost as old as AberMUD, but still appears to be actively developed nearly three decades later.
Flightgear Multiplayer Server is high bandwidth according to its installation instructions, but if you're only using it with a few clients on a local network then I expect it would be fine for practising formation flying.