We all have seen people who want to get to 90 ASAP, and are happy with less XP needed per level, or Heirlooms, or whatever gets them to 90 fastest.... And then there are those who like the slow, stop and smell the roses progression.
So, I am only putting this out there because this game is about 9 years old now, and losing its subscriber base, and probably most people already have multiple 90's anyway.....
Could Blizzard make it so you could get to choose for as yet unmade characters, whether you wanted to zoom to 90, or choose to progress slowly...... but once you decide, all future characters would have to adhere to your choice to 90. At 90 - 100, everybody would all have to level with the same progression. I reckon it "could" be done, in the same way, sort of, that current play takes away our ability to fly from 85 - 90 in MoP.
This might be a 100% bonehead idea, but can it be done on a technical level?
If WoW was new, I wouldn't even want Blizzard to consider this, but considering the falling player base.........