X-Piratez (openxcom mod about lesbian space pirates, haven't played it but looks like it has very deep lore and complex mechanics)

Warzone 2100 (RTS)

Battle for Wesnoth (TBS)

Mindustry (Factorio-like but more casual)

0 A.D. (RTS similar to Age of Empires)

Beyond All Reason (RTS similar to Total Annihilation/Supreme Commander)

openTTD (transport tycoon)