As far as I understand, at arena we will fight for money, fame, maybe slaves with random parties consisting of random and unique girls which didn't join MC yet.
And you have to sign up for battle in advance, a few days or even weeks before it.
Not sure if I have anything major to add to this concept, aside from various modes (for example, some kind of ladder, like in Mortal Kombat games). Maybe limitation on party members, so you cannot take 3 people to one on one duel mode?
Also we have to decide what to do with characters without battle sprites. I propose to block warrior occupation for them and use only warriors inside BE, including arena.