League's Lore has become cheap yet enjoyable trash, much like fast food, so I say go balls-to-the-wall, Hollywood style.
New map, HUGE hole in Runeterra opened and is pouring voidlings. Make it a Prey style game, in which four players unite to try and take a fifth player who takes the role of a Void monster trying to build an army of minions to expand the Void's influence, while the four players do whatever they can to close the Void Gate.
Each player has his own base in each corner where they'd start, and would try to attack the center stage of the map where the Void Monster would have to manage his defenses, much like Nexus Siege combined with Bilgewater Mercenaries, to try and halt the enemies while at the same time attacking each base. The Monster would be able to 1vs2, 1vs3 if good, but never 1vs4, making coordination absolutely essential.
Every time a player base was destroyed, the other players would reset back to their bases and get a buff making them stronger, while the Monster would get an extra resource point (the previous player's base) from which to build his defenses. Damage dealt to bases previous to the destruction of one sticks. A defeated player could exit the game or optionally spectate from a player's viewpoint.
The Void Monster player would win if he managed to take down 3 bases while the players would win if they managed to shut down the Void Gate.