An open source, smash style fighting game, it could use mascots from foss projects (just like SuperTuxKart do).
Since there are some SSB knockoffs being released (All-Star, MultiVersus), why not make one that is actually good? I think it would be pretty cool :)
Yeah, make Tux brawl with… those annoying assistant mascots Win XP used to have (like that talking paperclip, if anyone remembers)
Clippy!