If I knew how to mod, I'd make a weapon.. it'd be called "the cookie" and shoot giant explosive plush cats from a huge barrel made of swiss cheese.
Seriously though, I can't really comment seeing as the stupid retail code hasn't arrived yet.
(Damn you, Paypal!)
(ONTOPIC: I'd make a mod with more 'futuristic' looking bricks, that would probably get failbinned. Shiny half-sphere bricks from LEGO astronaut sets, anyone?)