Apart from one vendor, there are no official linux suppoerted mice. I think it would be really cool if valve designed a mouse/periferial "core", beeing a supercopact unit, with extentions to what the said peripheral ends up beeing. So its this super compact design, You hook up a sensor or more, like a laser sensors, and 3d-print the components needed to complete the assembly, Also running embedded linux for further custumization, and or to store your profiles. Has its own IP over USB, so that you could ssh in, and even run custom programs that change the behaviour of the device even further, and ofcourse update firmware etc.
But yeah a straight up valve-linux gaming mouse lineup, with open source configuration software, working on all OS'es or atleast has the greatest potential to. Something really compelling. And a relativly small project id think. Even if it doesnt turn out to be embedded with linux with 16gb storagespace and a development enviorment with an arm processor, and its own ip and a sshd. But that would be really cool =).
Also the idea of this kind of "core", would be something you put in all the peripherals, like the valve steam bigpicture mode gamepad, steering wheel, space sim joysticks, gun controllers with the oculus rift, maybe in the oculus rift itself?, or some kind of attachment?.