For starters, humanoid-motion legs would be a nice addition to the game, since building massive robots is part of it, so maybe a new propulsion system would be something useful, such as having heavy armor, being rather slow, and being very big. And maybe add robot arms, that aim a cannon that you place on the end of the arm. I really love this game, and massive robots are pretty cool! :3
Might be something neat if micro modules are ever added.