I've played Endless Sky and liked it a lot; although I'm pretty bad at combat.

Some other games: https://store.steampowered.com/curator/38475471-Libre-Open-S... https://github.com/Martin-Pitt/awesome-frontend-games (<- I haven't checked if these are FLOSS, but I had this in my bookmarks)

There are also many mods for games available in GitHub such as: https://github.com/cybrxkhan/VIET-Events-for-CK3 https://github.com/kiooeht/ModTheSpire