This is a very long CD summoner spell that after use, dramatically enhances one of the champions' abilities for one use. Preferably ultimates, however seeing how some champs don't have ultimates or have them hard to make over-the-top, it could be any one skill.
For example: Janna's R: Wind Barrier Double's the healing rate and makes the healing field completely unpassable for enemies until it ends.
Corki's E: More Dakka Lenghtens the duration and range and gives Corki 2.5 AS for the duration of the spell.
You get the idea - a very long CD summoner's spell that makes one of your skill over-the-top. (by LoL standards)
I realize it would take a lot of balancing to do, but hey, the game changes constantly so it's not like they are not struggling to make the game balanced already.
And since it has long CD and costs one summoner spell slot, I think it wouldn't be instantly "take-every-match" spell.