Know that is a bit of a shocking title but let me explain my thought process. Currently in mythic plus I think everyone can agree there is a major stagnation problem once players have an established group running 15 keys. Trying to get a group between 10-13 is like pulling teeth for new players.
A simple solution to this would be to make only normal mode queueable then buffing a heroic to mythic 7 difficulty and rewarding appropriate ilv then in turn buffing a mythic 0 to a mythic 15 and letting it drop relevant rewards.
My reasoning behind this being that if you squish the possible levels down between difficulties you would heavily cut down the "dead keys" that are created in the weird inbetween no one seems to want to do.
I would still have keys scale past that new mythic 0 difficulty for additional power rewards but I feel there is to much of a spread for the progression part of keys.