The Ultimate Arena - ALL VS ALL -

You mean one of every champ in the game or just a free-for-all deathmatch mode? If the former you're going to get some serious lag issues trying to maintain a server running all those players and entities at once, and if the latter it would probably only be good for a select handful of champs who can solo teamfights while the rest and especially supports or junglers meant to work alongside other allies would be severely underpowered if not useless outright.