Personally, I disliked that the enemies were scripted in Oblivion, the fact that you can "finish" by killing all the guys was not my taste. I also didn't like deciding when to fight, it seems like I am managing the arena, whenever I fight there is a fight, or when I want to watch one there is, and when I don't - there are just people practicing in the same place all day. Doesn't feel right.
I hope this is improved for Skyrim and you can always get a random fight in the arena with random foes, and there is more sense of a living arena with special events where higher ranking people also attend etc.
Oh and, maybe more arenas then just one?