My ideal expansion, systems wise, would:

[reploid]

Bring back artifact weapons, BUT without the skills, using the concept of the essences in heart of azeroth (which would unlock account wide). Reasons being: a) its an extra layer of choice and customization b) weapons have always been so friggin hard to come by that all my characters are always several ilvls behind their gear with their weapon. Plus artifact weapons added to the story, felt awesome to have and added more lore/ quests

Give back all skills all classes have lost since WOTLK. Yes, all skills. I dont care if some of them are sub-optimal, I would be the 1% that would choose to use one of them for flavor and fun even if that caused me a 5% dps loss.

No new classes, but focus on going back to WOTLK old style talent trees and redo every spec progress from there. Evolve into something with more talent tiers, more choices per tier and more customization

Give DH their well deserved 3rd spec. Possibly with a scythe (and finally make scythes a weapon type), more focused on mixing aoe sweep attacks and long range spells.

Remove race/class restrictions. Nothing to say on this one, just that after 15 years, if someone wants to play a gnome paladin, they’ve damn well earned it after sticking to the game this long. Create a toy called “Lore Buff” that someone can use if they feel offended by someone else’s race/class choice so that to that person, they see only race appropriate classes. (i/e to their eyes you’re a human paladin)

Undo all the harm BFA has done. Which means the change in armor only protecting against physical damage (and consequently allowing a lot of spells that were changed to physical damage to going back to their real damage type. Also remove the GCD changes, those are still awful to this day.

Give each class something truly unique. I am all for bring the player not the class, but homogenization sucks. So what if a fire mage has a 10% resist to fire damage, if I want to bring a warlock to the same encounter I can just buy a scroll that does the same thing. But the mage has that passive, its part of him, its part of what makes him special.

Reduce the duration of stuns and the effectiveness of stun chains in PVP and also stop with that nonsense that when you’re out of a loss of control effect you’re always with your back facing your opponent. That’s just op and unfair.

[Guildenstern]

Remove race/class restrictions. Nothing to say on this one, just that after 15 years, if someone wants to play a gnome paladin, they’ve damn well earned it after sticking to the game this long. Create a toy called “Lore Buff” that someone can use if they feel offended by someone else’s race/class choice so that to that person, they see only race appropriate classes. (i/e to their eyes you’re a human paladin)

Can I have this toy and not see Vulperas, please?

Other than that all ideas are good, except I wouldn't remove the stun lock which is pretty much a core part of rogue philosophy.