I consider that the arena fights get a little bit boring over time. Just the usual legendaries in the higher leagues and at a certain point almost no chance if they are not maxed out. Why not add a little more challenge?
I would propose that one player starts the fight and the other can pick the category of creatures to fight with. In that way there are much more different outcomes possible and lower level players that have good rare creatures could even win against those who only care about the legendary ones. All creatures would also be worth something in the arena.
What do you think?