I was thinking an arena could be cool. Its what I thought zom zoms was till I saw that it wasn't. would be interesting not to be able to flee or be ran from. How about fighting a dozen melonheads?
I think an arena could be good if it was done right.
Or change Zoms to be arena after your first experience. Spectators get gamble option, returning "victorious" stock get to call for one of their weapons when going in the pit.
Zom Zoms is the natural choice for an arena, but I don't think the robot would be survivable, thus some new things would have to be added.