although I've searched for specifically FOSS games in the past it's not really a niche, i would still pick out games in genres i liked.

still as you have said there's a real lack of "good" FOSS games, because most people make games with engines that do not facilitate open sourcing your game, artists unfortunately think copyright is good and you can't open source your game without also giving away your art or making your game free, or the most common case is that devs don't know/care about FOSS.

i think FOSS games are good however i also think games need to be very opinionated pieces of software, so do not delegate your design to other or "also open sourcing your design". the dev should ultimately be in control.

one thing I've seen even non FOSS games do is to use their repo as a public bug tracker, so in addition to that accepting contributions for bugfixes could also work. although i think it goes against the spirit of FOSS software you could also separate your artwork from your source code.