Where players can "Pay" to:
Fight eachother for fun: and not "die" for real.
Duel for Equpment: winner winns random worn thing from opponent. Maybe small chance of getting carried item.
Duel to Death: One player exits arena, the other is Nuked. >:(
Duel equally: Players are made same level (or same Exp)(average maybe) and naked when entering. Only books maybe. Standard equipment of choise from armoury. And then you face eachother.
Mass Duel equally: For gold, with maybe 10 players :P where winner takes half entrance fee from all participants. ;D
Beat hiscore for fastest cleaned pit with monsters. One hiscore for each type and maybe a few deepth.
Monsters that dont drop things and Exp you dont gain. And spectator seats need to be safe

Once player(s) exit grounds he is restored to original "shape". and maybe an extra item or gold.