Spent a lot of time in a dark house thinking about this thread after Helen, now that I'm getting back up to speed I figure I'll share the weird, homeless games I tinker with in my quiet hours.
Perfect Draw - A PbtA game mostly inspired by Yu Gi Oh!. I always say that PbtA games are best when they can strongly evoke the genre that they're emulating. Perfect Draw does this by having the PC's build a small deck of cards (like 4-8 cards) supplemented staples that can be played repeatedly, with dice rolls substituting for quick-play interactions and the like. The GM, by comparison, sets opponents up as more of an "Encounter Puzzle". The whole design space intrigues the hell out of me and I'm hungry to bring it to the table.
Alas, my tabletop group can't play card games without being toxic as hell, and I need to figure out how to simulate the deck draw components before I can try to pbp it.
Sentinel Comics - Ran a campaign of this for my tabletop crew and I love it to pieces. 4 Color Superhero shenanigans, the way the game mechanically incorporates environments and tension clocks allows for me to do more with scenarios that aren't just a fist fight. It's just nice to have a game where little boosts and penalties matter, and where having someone in the background handling the non-fighty bits can have as big an impact on the scenario as the combat guys.
It is, admittedly, a little gamey, and the character creation can be a bit tricky to get your head around. Fun though.
Grimdark Future: Firefight - Admittedly, more of a tactical wargame than an RPG, but I saw Appalahi's Narrative Wargame Interest Check thread a bit before Helen and it's been rattling around in my head ever since. I'm more of a Sci-Fi person, admittedly, so that particular IC check wasn't for me, but it's good to have the inspiration all the same.
Ideally, it'd be a fairly short campaign. Something like 6 players running through a 5 round Round Robin tournament, with Campaign rules allowing the warbands to develop over time. Put together a narrative that runs through the whole thing. Might use Roll20 to handle the table, hexes substituting for inches, especially now that Roll20 supports putting players on different pages at a time. Alas, I'd probably have to relegate myself to a GM/Referee role throughout, but it'd be fun to facilitate.
Biggest thing holding me back is that my map making chops are extremely rudimentary, and I'm not sure people would enjoy playing one the tables I'd be able to create.
Age of FATE: Tabletop 4X - This is a bit of an on-again, off-again project that I tinker with in the background. Largely inspired by Age of Wonder and Legacy - Life Among the Ruins, the idea is to create kind of a hex-crawler with each space being a FATE location with aspects and such. Each player plays a Faction Leader with full Stunts, Fate Points, etc., but can build lesser units, and as each player explores the setting they'll bump into each other and vie for resources, strike up alliances, and all that good stuff.
Leaving aside that I keep tearing the mechanical end of the whole thing apart and rebuilding it, I'm not actually sure there's an audience for something *that* zoomed out though.
Multiversal FATE - Thing about me? I love a genre mash-up. So, inspired by things like Lords of Gossamer and Shadow (never read the books Amber is based on, which has served as a bit of a barrier to entry), a game of vast interdimensional conspiracies perpetuated by plane jumping immortals has been on my back burner for a while. To facilitate the broadest number of character concepts, a generic toolbox system like FATE seems best. This is one of those game ideas where it'd be important to get those "Crossover" Aspects to tie the PC's together, but it'd be pretty wide open for character concepts. PC party of a Victorian vampire hunter, a reincarnated slime, a wild mage and mecha-Lincoln? Sure, why the heck not?