Rattlebones was good fun, and fortunately, other than the interchangeable dice, had nothing in common with my game. So I put pen to paper. (Or keystrokes to a keyboard.)
The birth of the idea for IMDL!, which actually hasn't changed through the many versions, was:
Enemies invade earth
Players take the role of a Mech warrior
Players upgrade their mech and corresponding dice
Players kick enemy butt.
Rinse and repeat against harder enemies with bigger and better mechs.
I knew I wanted an international feel, and I knew wanted to see the mechs grow. How that was going to work took the next couple of months to figure out...