There's 2 types of Horizontal leveling btw.
The "build a character" type, which is what most MMO devs dont want. (and most horizontal fans do)
And then there's prety much what we already have, except no power creep.
Example:
Once you reach max level, lets say you have cheap starter gear Lv70, this can be bought/farmed.
Then you have semi customized class gear. Generally similar stats, but closer to "BiS", so the difference is minimal, and technically the same iLv.
This "part" needs to be repeated on every job you reach max level with.
Then you do the current endgame content to unlock gear for that content.
Example:
Deltascape gear adds "Does extra damage to all enemies within the deltascape"
Next patch, Sigmascape comes out, and all gear adds "Enemies within Sigmascape take more dmg."
But the base stats are identical otherwise.
You can use your deltascape gear to enter into omegascape, or you can use your job quest gear to enter (unless we flip it, and bought gear is the starter gear)
Omegascape gear offers... you guessed it "enemies within omegascape take more dmg"
you get the SAME result we currently have, but NO POWER CREEP!
(Its not the horizontal progression people want, but it DOES make all content equally challenging, and feel equally viable to everyone honestly.)