I love Hextech Crafting, but there's room for improvement.
The biggest problem I see is the reversal of importance the chests and keys are going to have. In the beginning, it's gonna be super easy to get chests, so you'll have an abundance of chests with no keys to open them. Then as it gets harder to get chests (the more champions you get chests with the less champions you have to get chests with) you'll have a shit ton of keys with nothing to open.
My proposal to fix this is making it so that if you get an S rank with a champion you can't get chests from and you win the game, you get a key or key fragment instead (Or even make it so S- gives a fragment, S gives 2, and S+ gives a whole key). This way there's a reliable way to get keys, but it's not easy to do. Then for the chests, every 5th S ranked match with a champion that you can't get a chest with gives you and only you, a chest.