Dispel grenades sound awesome! I love it. Funny, only thieves have throw? I don't know if you're using that skill for the nades, even.
Just a thought, the shop you add doesn't need to be all new items. Plenty of potions and scrolls and coconuts around that might be useful in an arena. It might be good to disallow people from bringing in potions / scrolls from outside the arena though, and force the teams to purchase those items from their team gold.
Sounds like fun though, I hope to try it sometime.
