Just make a buff that would make any champion considerably OP. And then the next poster has to balance it out without removing the change.

Example:
Person 1: Darius's ult is now an instant kill. no questions asked.
Person 2: But his ult can now only target minions.

For mine,
Zed:

Death Mark - Now creates 10 living shadows behind the target he marks.

But the shadows do absolutely nothing now and is only for visuals.

Draven now always has a spinning axe ready.

But he is stunned for for 1 second each 2 seconds because he gets too tired.

Teemo can now place his mushrooms globally.

But they are capped at 100dmg per shroom

Master Yi's auto-attacks now scale with AP

But the only thing it does is poking the enemy for 1 (+0,0001 AP) damage.

Sona:
> Power Chord now has a +2 AP ratio and only needs one cast to be used.

But power chord now has a cooldown of 30 seconds

Udyr's Phoenix form now grants him 500% MS and lets him go through walls. (He flies)

But Udyr has a 80% chance of getting stuck in said walls.

Zilean can now Rewind games entirely.

But it costs him 100% of his maximum health and causes his respawn timer to last for the rest of the game after being used.

Nasus' Q is now his auto-attack.