I just played Xonotic for the first time ever and now I'm interested. I like a game I can be sure isn't selling my data to China.
I would personally like an RTS, a game in whatever genre Stick Fight falls under, and sandboxes, although any genre is fine. I don't contribute code, but I'm pretty good at finding bugs, etc. and get a few laughs out of people on the forums with my jokes.
Games in early development preferable, mostly for the active community these tend to have, so I can share less formal work with them and join their Matrix.org room or something
0 A.D. is a modern take on Age of Empires.
Other than that I'm aware of a bunch of projects that reverse-engineer engines of older games. They're FOSS but you need to buy the original game in order to obtain actual game data (models, textures, maps...). These include OpenRCT2 (Rollercoaster Tycoon), OpenMW (TES3: Morrowind), and Freespace 2. I just noticed that Freespace even has standalone games now, so you can play something without buying the commercial game, but this is the first time I'm seeing that so no idea about the quality.