A gamemode where everybody starts at level 18, waning down back to level 1 at timed intervals. It forces people to finish early, punishing players who troll or just want to have fun. It symbolizes the battle with cancer IRL, and should thoroughly and effectively prevent anyone from ever calling true League of Legends cancer ever again! I'd release this for the next cancer awareness month. Yes the "late game" of this mode would suck, but that's just the thing. Cancer is loved by nobody. #DealWithIt
Aside from that idea, I also had another LoL minigame in mind where it's just normal League but every time you right click, you flash in the direction of your cursor. Imagine how fun it'd be to try aiming skillshots and executing strategies if your character is always flashing. I'd call this mode Flash Happy. It'd be funny to see people play this mode, then it gets taken away, and they struggle the first game or two back on normal Summoner's Rift NOT being used to flashing every click.