Three player is selected at random to be The Big Ones. The selection happens after the players have already chosen their champion.
The Big Ones will be a larger version of the chosen champion, and with enhanced skills.
For example, Yasuo would be the april fools version but with a less broken ultimate and have increased stats (mostly in health and resistances).
Teemo could be a gigantic version of himself and his shrooms can be even deadlier explosions, his bouncing mechanic bouncing even further. Teemos darts can pierce through enemies.
And so on...
There will be three Big Ones who will be defending their side of the map.
There will be seven regular champions BUT these champions will be able to augment their abilities in the shop with gold as the games go on, like in Odyssey. This will allow them to have a fighting chance.
The aim for both sides is to destroy each others Nexus like normal.
The map will be augmented and be different to the normal summoners rift.
Other Big One champion ideas can be Heimerdinger with amplified abilities. Being able to spawn more turrets, the rocket barrage having more range and so on.