Here are some ideas
It would be cool if the game notified in system chat when one player defeated the other in the arena example: Player1 defeated Player2.
or when the player wins the Duel, in addition to showing in the system chat, a notification balloon of Winner appears on top of the character.
Notices around the arena showing that someone is defeating many people in the arena, for example: Player1 has defeated "number x of players" and is dominating.
It would also be interesting to gain points by participating in the arena, the number of points won could be based on the level of the player, if the adversary defeated is very low level beyond yours, you will not receive points, and so there is no free kill, could have a negative effect on dying.