they can have arenas only if the devs change nothing from the game to "balance" class to appease the arena players. also arenas dont have to be solely pvp you could add in something like this https://www.youtube.com/watch?v=3FmyK-ips2g
if you played any of the mini game arenas from the guild wars games that could be a good way to have arenas as well. i'm talking about where you were only given certain skills and it was more about doing something faster/better than your competitors than killing their characters before they killed you