Some great suggestions in this thread already. What about some of the
games that Loki Software previously ported to Linux. I'm not sure who owns the rights and source code to these games, but there are some diverse and high-quality titles there like Alpha Centauri, SimCity 3000, Decent 3, etc. And since Loki was able to get the rights to do these ports, chances are, the original owners are at least willing to talk.
Unfortunately, Scott Draeker of Loki games also shafted many of those studios and publishers to the tune of millions across all of them at the end. My take on the ones you've mentioned:
Alpha Centauri - Maybe. Need to talk with Firaxis about it since it's their baby. I'd love to get permission for a respin via any publisher (LGP or GoG) for Linux and Pandora for that one.
SimCity 3K - EA. Forget this one. Serious.
Descent 3 - Maybe. Interplay's got Descent available via Good Old Games for a $5.99 download. We could talk with them on the subject and see if they've got the port code. If so, it's a front-runner for things just like Alpha Centauri.