So, there is a feature they could include that could be interesting. It's not really "original" and if you think it's actually bs, it's fair
This feature would be to incentivize people on doing high level M+.
The legendary gem system from Diablo 3.
Make legendaries have their socket to be "only a legendary gem can go here".
Legendary gems drop from 3-affix M+.
Legendary gems have ranks. 50 main ranks. Every 5 levels the gem gains 100 main stat (or an specific secondary stat). At ranks 25 and 50 you could add a unique bonus of that gem.
Every level beyond 50 adds 5(?) to the stat. Unlimited ranks.
Legendary gems can be swapped from the legendary item without destroying it.
Different gems with different bonuses to choose from.
Completing a 3-affix M+ gives an option in the end to: Upgrade keystone or gain 1 rank on the gem.
Per 2 levels above +10, increases the chance to make the rank upgrade more powerful, making it proc by 2 or 3.
Don't know that to expect from people's opinions, but well, here it is.