I'll go for the red koopa shell.
Because they would go nicely with other mario kart stuff.
Also, one use disappear please. :)
Abso-freakin'-lutely!
Here's what I would like:
A side-view gamemode (for all the Mario needs. I think Space Guy made one back in I think V8... but I haven't seen it since.)
AI (Goomba, Koopa, Boo, etc. That way Mario & friends have crap to jump on :D. You could also make friendly AI such as DK.)
Starman (You could make it so that the player glows when they use it, and they can't be damaged. (A playertype that can't be hurt would help you on that.)
Maybe change the ordinary jump to the Mario jump sound.
As much Mario music as you can get your hands on!
I know I'm asking a lot, but I could help if I knew how to make addons.