His idea is basically that each expansion would be a self-contained game branching off of the end of classic, with no expectation that you take any power gains with you from one expansion to another. Top-of-the-line Legion gear would be no more powerful than scrub end-of-classic gear once you step into any other expansion content. If you earned any class abilities during your time leveling in Mists, they would only work in Mists.
Isn't this more or less what they did with Legion -> BFA and it sucked? The Artifacts gave interesting abilities and traits and progress, but then as soon as BFA rolled around it got wiped out and you got shit-all to replace it? I think there's merit to that approach as a way of making legacy content interesting for new/returning folks, but as a long-term sustainability thing, it seems like it's just going to exacerbate the existing problem of "Leveling ultimately doesn't matter. Nothing changes. Anything cool we
do get will be removed anyway." Might work better if this opens the doors to more significant and frequent power gains within each slice, but I'm not sure that necessarily outweighs the more frequent "I lost all my new cool shit." I feel like I've played some games that had areas where there was a separate progression tree that only affected that zone/area, but those games were substantially less linear than WoW where you hit Raid 1 to be able to clear Raid 2 to be able to clear Raid 3 et cetera. Maybe this would work better if it was immediately replaced by an equivalent system. Would the abrupt loss of all the Legion artifact stuff felt less shitty if we got a new artifact weapon right off the bat in BFA?
I actually think the perk system from Warlords was a potentially good idea for making leveling up matter, minus the random order part. Back in the days of yore, a lot of the "stuff" you got from leveling was just new, more powerful ranks of spells--but at least it felt like something. It does eventually run into the same issue of only being able to layer so many advancements before it becomes unsustainable--You can't have Sinister Strike do damage, cause self-healing, cause a debuff, increase dodge, reduce energy costs,
and generate bonus combo points without having tooltips take up an entire screen--but it gives a way to have continued minor progression that rewards leveling without potentially dramatically altering class rotations by introducing ten new buttons.
The past few expansions have seemed to try to reinvent the wheel with a brand new design each time, and it hasn't been a great transition most of the time. I wouldn't trust them to do a more extreme version of that any better.