Stardust

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.

I definitely prefer here to Mastodon so far, simply for that reason.

There are several starter designs past Gen III that I never really cared for. In Gen 4 I tend to just dump my starter.

I've started to dislike Zacian. It basically ruined the Pokemon Unite moba game, which was for a brief period actually sorta balanced and not-pay-to-win (except occasionally an overpowered mon for like, two days before it got patched). Zacian ruined all of that. Even after it became available to freebie players, it stayed horribly broken. Admittedly it can be very satisfying to gang up and murder Zacian, but if it gets any kind of lead it will just absolutely delete your team. Likewise I don't like the eevee-evolutions as much as I used to as the developers over-favor them and kinda spammed them. My understanding is that she (Zacian) is broken in whatever meta she enters, so this isn't unique to Pokemon Unite which is tbh a shit game that could be fun if the devs weren't jerks.
On the plus side, the game did make me like Greedent, Zeroara, Eldegloss, Cramorant, and appreciate Mr. Mime who has a really interesting play-style and has saved my ass a few times. Thank you Mr. Mike.

I don't really like most megas. I will ditto hating rotom.

I love original Raichu because I feel no one loves him. He's forgotten boi, replaced by his cuter cousins Pikachu and all of Pikachu's clones. After that is Meowth and Mewtwo because they had personality and backstories in the anime. Butterfree. Aggron. Rattata. Eldegloss, Cramorant and Slowbro. Wooper. Victreebel. I used to dislike some of the stupider/uglier looking pokemon but they've grown on me. Like Bidoof.

I like most of the bug pokemon. The original starters and Cyndaquil, Totodile, Mudkip, Treeko. After Gen III I took a break for a long while.

I feel like they botched open world in Violet/Scarlet. What they really need is some kind of difficulty options, like level scaling. I've done some hacking and it's honestly really easy to make a toggle so that enemies are scaled to near your party and bosses slightly higher, or not at all if 'progressing'/grinding exp is important to you.
Some interactions with wild pokemon that aren't just murder, catch, or die to. Arceus experimented with this a bit in that you could feed them or accidentally wake them up, but I feel it could go further, like aiding one being attacked by another pokemon.

Game boy emulators work pretty great and with 'open source' versions of some gb game code (old Pokemon games mainly I think) many great hacks to improve the original experience exist. They take up an incredibly tiny amount of space, being old school.

Communities should have categories/hashtags that users can optionally sub to, like the 'metacommunities' like plz1 said but optional and multiple. Mastodon does hashtagging and can be done on a post by post basis. The forum software Flarum has a 'tag'/category system and an additional hashtag system, so what I'm thinking of is more like the Flarum system since it would be awkward to hashtag every single post in a community/magazine/whatever.

So if I wanted to just get solarpunk tech I'd sub to that, but if I wanted that and even moar I'd sub to a generalized Tech tag. Make sense?