Hotseat + helicopter

Why not?

Friend mode would mean that both players take turns building models, and then try them out side by side, in races, tasks or head-on collisions.

Darn them to Heck would be head-on collision only, turn based and with match after every 2 turns. I can just imagine me smashing my friends car to pieces, and afterwards him upgrading his, so that he makes it even, and so till we run out of beer and snacks.

Turn based, of course.

Oh, and one that would be single player mission - Helicopter level. Target jump level, just with rule that robot can't hit ground in the end - that would force player to build helicopter that can land safely. Why not?