Mastery Shards

For what? If they started giving essence there'd be a huge incentive to NOT level up to mastery 7 on your best champion and just repeatedly disenchant the shards...

Perhaps if they made it possible to continue earning them post mastery 7 as a unique reward, but even then it seems like it'd just be too much free stuff since it's guaranteed rewards for something you can do repeatedly, unlike the rest of the system.

How about getting some blue essence for high mastery grades? S- gives 15 S gives 25 S+ gives 50

It's a smallish amount, but if you play often enough and are able to get high scores, you should be able to unlock it fairly easily. It incentivizes you to play the champ often. It even allows you to build essence up by continuing to get high scores on champs you already have at lvl 7 (which is basically what we do already, play to get IP to get Champion Tokens to disenchant).