I'm starting this tread because a lot of other threads on different topics become about alternatives to levelling in MMO game design and would like to see those topics stay on topic.
I, for one, think that the idea has merit. It is untried and I'm not sure how it would work out, but I like the idea of not having areas of content become obsolete due to everyone eventually being too high-level for some areas and players simply power-leveling alts. Progression in this case would have to be horizontal in nature and there are many possible routes to produce this. I, personally, know of no game that has not had at least some form of vertical progression. Ultima On-line was skill based, but in the end, you had a main skill that you needed to level to grand-master and that was your class and your "level". Everquest Next claimed to be about collecting various classes that one could switch between and even combine to form your horizontal progression, but that game was not to be. If Day Break is to be believed (my trust isn't high) it was not fun in the end. But that doesn't mean that a game can't break the mold.
That being said, I also am fine with the levelling model. It works. It has its issues but it is tried and can be tweaked for maximum enjoyment.
I think that since Pantheon is already well into development with a levelling system in mind, their is no reason to change this. A good balance would be to make the power curve ,per level and overall, very shallow and have a lot of alternate horizontal methods of progression to round it all out.
Edit: Turns out that there is already a topic on this:
https://www.pantheonmmo.com/content/forums/topic/2820/character-progression-how
I guess I'm just frustrated with topics on zone restrictions and considering mobs, devolving into how bad leveling is.
I am glad you found the thread on this topic, my friend, I know it can be frustrating but the game has been in development for roughly 3 years and almost everything has already been discussed or mentioned at some point. Any post prior to 2015 was archived but the search function will usually provide you with what you need.
I will close this one down now but I appreciate the effort to search, it helps the community and myself out a lot, so thank you :)