I spent several years on OpenBSD playing mostly foss games (no WINE support) before I got my steam deck, so I feel fairly qualified with recommending Cataclysm DDA, Xmoto, and Dungeon Crawl Stone Soup. Say goodbye to the next several months of your life. Other cool FOSS games that grabbed me, but not to the same level: Ur Quan Masters (Star Control 2), and endless-sky.