As a ranking arena, one person bets the other with keys and challenges the other to play a level with the difficulty the other wants to do also sets the same number of keys (in the level there will be no keys) Both players finish, whoever has more points wins and gets all the keys for both bets
There are already Dares in the recently added League. They work the same as you described except you dont get keys other than the ones you collect in-mission.
It would be a nice addition to league, but it may end up raising the game’s rating if it gets one. So it depends on iA if he wants to deal with the legal mumbo jumbo or not.