I actually have my own science fiction universe written up for the Space Rover comedy drama audio series I do with my brothers.
http://icculus.org/mwm/rover/
I have a whole lore bible for it, although I have not published it publicly.
I came across this article last year on libre characters that seems relevant.
https://web.archive.org/web/20230212072149/https://blog.alexgleason.me/free-culture-fighter/
There is also Surge the Rabbit from OpenSurge, and OpenArena and XEvil both have some character profiles for their various combatants. As I recall the Parallel Realities actually had a bit of plot.
And of course the classic two lore heavy FLOSS games are Battle for Wesnoth and Vega Strike.