Probably an unbalanced but fun idea.

A game mode where everyone has 0 cooldown on abilities, put in an arena with no nexus/turrets (kind of like the dark star mode) without being able to buy items and the goal of the game would be getting a certain amount of kills/points(kills giving you the points).

IMO this would be pretty fun to play and at the same time would obvisouly not be too balanced but maybe add 10 bans as well to try and ban the really unstoppable champions.