To be honest In terms of class design what whould be needed are;
A End to external character progressions. Even if the Legion artifact wasn’t bad and to be honest the HoA on PTR seem to turn out fine aswell. These sort of progression measures make the classes balanced around “best possible setups of external sources”.
No class/spec felt complete in Legion before you “at the very least” unlocked all the golden traits. Just like classes don’t feel complete before you have the correct traits(for the occasion) in BfA. This system also affects talent balancing, where some talents become superior given different traits etc.
Another thing which could be usefull are a full breakdown of the current talents. The devs need to go trought all of them and see why some are picked over others, due to either them being significantly underpowered, messy to use or just don’t reflect a need the class have. If a talent are widely accepted as usefull in any of the major three criterias; PvP, Mythic+ or Raiding, a talents acctually fine as it is. But there is some talents who are not considered in any of these activities and these need to be replaced/reworked or rebalanced.
So my solution to these issues whould be to not incorporate a Item grant X power system in the future and instead focus on that classes are defined by internal class choices. Rather then external choices of equipment being at the core of a characters progression. Sure gear should matter, but at the same time all classes and specs should feel complete without a Tier set, correct traits or being at the planned end of a external progression system. In legion for example alot of classes was bound to; Having the correct legendaries and all the traits in your artifact.
It don’t mater if this system are bound to a Paragonesque sort of gameplay(like AP/Azerite) or to level itself. However It needs to be a system that functions trough all content in the game. If people whould be able to for example enhance themselves with Azerite related stuff from day one of a characters leveling process it whould make leveling feel alot better since you can set up goals for yourself.
I will use the skyshard system from TESO just to state an example. In TESO you can find skyshards that grant you extra skill points. These are aviable at any point in the game and if you are quite adventurous you can build a very solid amount very early on. I won’t mention the paragon system present there since it caused some great issues by itself, especially in low level Cyrodiil pvp.
Anyhow the point I’m making is. Any system in place need to be bound to a characters personal progress, not be removed upon every single expansion aswell as it feel impactfull enought for players to go out of their way to get hold of it.
The essence system in 8.2 sort of feel like steps in the right direction, It’s impactfull, takes you places you may not otherwise chosen to venture etc. However It also suffers alot from the fact that “it will be taken away in the next expansion”, It won’t be aviable before your at max level and some classes won’t feel complete without the correct essences and traits. It’s still an external system, which is less rewarding then a Internal system.
If for example the artifact system in legion instead was; Artifact weapon + relics, relics increase traits as usuall. But the “progression tree” instead was tied to your characters specs and the Artifact power was something else such as perhaps a spiritual titanic energy source that enhances your characters capabilities.
Then the system from Legion could had remained and the new Azerite system could simply enhanced players even further. The system could also been streamlined in BfA so players could now find the spiritual energy anywhere in the world due to it leaking out trought the wound.
Then you whould have a progression tree that functioned from level one, which whould in turn make leveling a more worthwhile experience, since you whould unlock points to spend.
It whould also have removed the issue of losing significant power going into the next expansion, kept the balancing work from Legion somewhat intact and the devs whould had saved themselves from re-inventing the whole system. It whould also make all the work done trough Legion feel less of a “waste”, since the Artifact points unlocked whould not have dissapeared.