Today I came up with a potential idea for mastery level 8 & 9 and I would like to share it with you.
For mastery level 8, the reward could be being able to evolve any champion's skill which is not the ultimate (Q,W,E, even passive). For example, an Ezreal level 8 could evolve his Q, W, E for longer range/more damage/ % of true damage/ reduced cooldown/ insert any ideas here.
For mastery level 9, the idea would be the same but applied to the ultimate, that is, being able to evolve a champion's ultimate. Take Janna's ult, with a potential upgrade of increasing amount of heal/ wider range/higher knock-back range/insert any ideas here. The idea being to give players the chance to customize their favourite champions as well as making them feel they do master the champion.
As for what it takes to achieve this, I have not yet thought about anything, but it should be hard to get it. This idea probably needs to be polished, so feel free to share your opinion.