http://youtu.be/6ZXGlQPIPk8
This is the idea: Coins are dispersed on the track (like nitro and gift) and as the other items, they respawn.
If someone has 10 coins (You can’t have more than 10 coins), you go approximately 5-10% faster (The difference has to be overall small) but if someone hit you, you lose most of his your coins on the track. (These do not respawn but other player can catch them

10 coins --> effect
Less than 10 (even 9 xD) --> no effect
Hit by:
Rubber ball: -6 coins
Bowling ball : -4 coins
Chewing-gum : -2 coins
Cake : -5 coins
Swatter : -3 Coins
Plunger: 0 Coin
I've made a picture with gimp to illustrate a possible coin (very fast ^^)

That's just an idea, i know you have other priorities
