I’m one of the 5 poor sobs who still plays darkorbit, and someone on github made a linux native launcher available as an appimage. I don’t recommend playing it though, because aside from the community becoming smaller and smaller, the game also runs like ass even on a high end system (not because of the unofficial app, but because of the game itself), and it has this issue with it taking up more and more ram over time, and it has been like that for over a decade now. Also a lot of server issues lately. The devs don’t seem very interested in improving the game.

Not sure how good it is but Ryzom is Linux native and open source. I’ll be checking it out soon

You could try RuneScape 3 or OSRS don’t really play it anymore but I remember it works on Linux natively.

FF14 works great with Proton. I would still advise to use XIVLauncher, as it is better in every way to the official launcher.

ESO runs great using Proton