This is just an explaination of why I don't like the Mech one.

There are plenty of Mech games out there already by big developers like MicroSoft. While a mech game would be cool...it's just not original enough to get interest. A game like the pencil one would be a game that is definitely new and original and gamers could pick up on without declaring it "just another robot game." But, then again, making gameplay could be difficult. The robot one would probably be most easy to make into the open source format, as the person could edit weapons/robots and such. Still, I like the pencil one most.

Oh, and whenever I see "Sketch", for some reason, I think Skitch. ;-P