Not sure if you have already thought of this, and whismicalyuki has already said part of this, but i have several ideas - Item System:
A new item system for bosses (if implemented) and perhaps something to do with honor (that is why you implemented honor right?? or did you just leave a pvp for a record?)
A new currency/item system with perhaps a vendor to accompany it? say if you beat the arena you will get points relative to your level (i recommend a exponential system) with items at different levels about equal to chaotic boss/boss levels.
Changes to the Arena Style:
Some of these arnt necessary, just for aesthetic value.
A new way of creeps entering instead of just being teleported to the middle (ie more staged - monsters walk out, and go to positions while invulnerable not attacking obviously, then turn vulnerable and are set free of AI, maybe also a pathing blocker preventing players from moving behind the monsters)
A final boss at the end instead of just a few creeps, Perhaps having a chance (any?) to drop a boss item/chaotic boss item
Make the Arena Entrance Grand, as in make it stand out more from the town
Mabye a storyline to some of it?
I know some of these suggestions would take alot of time to incorparate, but i think some of them would be great in later versions