Please, lord, no WoW-like "arenas", wow lost the meaning of arena's and took it to the extreme.
In such a medieval world, you would expect a "coliseum" like fights, not a hardcoe, ranked system for 3v3, 5v5, or w/e.
I wouldn't mind some arena battles organized by sadistic "merchants", to throw in gladiators, but thats it, it shouldn't be a automatic match setup, like dungeon finder in wow or arena queue.