Legion is well done and I like it

In fact, I play it a lot right now. Already 3 days /played at level 110.

Now, there’s a real possibility that it can’t keep me long-term, like every expansion since TBC could not. But however it turns out, Legion has very much been worth its money. What Blizzard calls "class fantasy" - classes having diverse strengths and weaknesses - combined with rewarding 5-player content is a huge improvement.
Enough of the useless praise … How could it be better?

The Power Scaling!
Sometimes someone is just about to find the solution and then, one inch in front of it, he stops. That’s what Blizzard did with scaling levels and PvP.

See, in Legion, your level doesn’t increase your power, it only unlocks content. Well, Legion tries to sell the illusion to you, that your level increases your power, but it’s just that: an illusion. And an unimmersive illusion as well, as the truth is not even well hidden.

All mobs scale with your level – so why have levels, except to unlock content and skills? ..
The same with PvP: When you enter PvP, the amount by which your level and even your gear increases your power, is compressed by a factor of 10. Ten!

Let’s turn this around: Legion has proven that players don’t need to grow in power nearly as fast as Blizzard has thought in the past! That’s great news!

Hopefully, the next expansion will have ways to unlock content and character skills (formerly levels) and characters that gain power at one 10th of the speed of today (todays PvP power scaling).

What are the disadvantages?
The illusion of super-fast power growth, that everyone with half a brain can see through, is gone.

What are the advantages:
A super-complicated system is pruned away.
No special rules for PvP required. You build one character.
A more immersive world, where last year’s gods can’t be one-hit by a stray arrow.