Here's an idea:
A really simple bot that users can talk to via the fediverse and will spit out responses if talked to, like a roleplay bot. I suggest this because it probably wouldn't take much time to complete basic functionality, being purely text based, but could be easily expanded to make the gameplay more interesting over time. 'Multiplayer' could be done via a command like '@roleplaybot I attack @Stardust' (parsed by splitting/exploding the string) and having some kind of simple database storing HP and such.
Then if that works, maybe a more ambitious version of the game that actually displays images and such, perhaps 2d.
Example commands:
@roleplaybot generate me a dungeon/monster/goblin
@roleplaybot role dice d6
@roleplaybot DM a session between me and @Stardust
@roleplaybot Set my class Rogue Level 1
@rpbot Travel up stairs

Could be a lot of fun. You'd probably be looking at using something like javascript or typical webserver backend like php, ruby, some of the already existing fediverse code you could just start modifying. Take one of the existing scripts that do something on an activitypub response and include the roleplay bot's stuff. Nodejs seems like an obvious choice for a game + webserver but I'm not sure how much activitypub coding exists for it, haven't looked.