Try Again - The Game Breaking Ability?

I was brainstorming abilities for a champion concept and I thought of one that I doubt we will ever see:

What if there was an ability to go back three seconds in the game? How impossible is to program something like that? What if only the champion, that casted the ability, could change their actions for those three seconds? How unbalanced would that be? Why can't Zilean do that?!

What do you guys think? Upvote if you would love to see this kind of ability in the game. Downvote only if you are going to explain why.