An Idea for the Training Mode

So recently Riot said that at some point there will be a Training/Sandbox mode. I thought it would be an interesting idea, depending on how the mode works, to allow players to rewind the game in order to make different decisions and see what they could have done better or what they did wrong. I feel like some things happen so rarely and are so few and far between that it's hard to really get the instinctive reactions down correctly because it's so difficult to practice them. If players could rewind the game and make different decisions or even the same correct decision over and over to build up the reflex, I think that could be extremely useful.