Here's an idea to get your brain juices sloshing:

Planet Mechaius was once populated by a race of super-genius beings that were really into technology. Through the years, they continued replacing parts of their bodies with mechanical utilities until they finally reached a point where they were no longer biological, but purely mechanical.

I'm thinking most of the mech characters would be MechWarrior-ish, but really any kind of machines would be possible.

The environment would be mostly mechanlical and indoors, but there would be lots of room for outdoorsy stuff too.

-Onag