I agree with Pickle in that open sourcing their back catalogue, such as Half Life, would be awesome. Also, improving Linux game development tools would also really help things. Anything that makes game development easier and fast. As an example, a Linux port, or an open source release, of the Valve Hammer Editor would be really helpful for game devs.
Sometimes I look at Unity3D and wish we had a Linux equivlanet. Unity3d's strength isn't is power, but how conducive it is to rapid iteration, of getting something up and running really quickly, and supporting fast changes (I guess Panda3d is a bit like it, but you see what I mean). There are a lot of development tools for Linux, but ones that make game development easier and faster would always be apprecaited (look at what Ren'Py has done, to give an example).