Thanks to the masses of people for letting me know. Undertale, the 2D RPG where you don't have to destroy anyone is now available for SteamOS and Linux.
Cubway is a journey with an abstract story of lifecycle of rebirth, called Samsara. Guide the cube through the long way full of dangers and difficulties, visit many interesting and mysterious places.
I've enjoyed a great many hours in RimWorld. It has charming visuals much like Prison Architect, but you're building a colony with a choice of different AI behind the scenes.
Tahira: Echoes of the Astral Empire, a tactical turn-based RPG in which a 20-year-old princess must save her people from a genocidal empire, launches on Linux on August 31, 2016.
After some delay, the latest DLC and patch for XCOM 2 is now out for Linux. Shen’s Last Gift adds mech units to the mix, to better fight the insidious alien threat.