Seen a great post on Reddit ( by skilliard4) regarding 3 things MMO's get wrong and the solutions to that. I think it specifically pertains to WoW for sure.
Here :
1. Abandoned Content
This is especially a problem with themepark MMOs. Developers make these massive, beautiful worlds, only for players to never return to 90-95% of the content after experiencing it the first time.
There's a problem when endgame consists of playing in 2-3 endgame zones, while the other 30 zones serve no purpose but leveling content.
It just makes the world feel so small, and games get repetitive as a result. I can't find myself sticking to an MMO when endgame is just running the same 2-3 raids every week. Overall, this design results in a huge waste of high quality content.
2. Lack Of Rewards:
A lot of times MMOs will create fun, unique gameplay, but give players no reason to play it. Many times its more efficient to run boring dailies than go on a challenging raid. Challenging content should be heavily rewarding for more than just gear, it should be profitable so that people don't stop running it after they get what they need.
3. Repetitiveness
Perhaps the #1 reason people burn out in MMOs is because they're tired of doing the same thing. Log in, do daily quests, dungeons, and raids, maybe garrison/farming system of some sort, log out.
Designers should find ways to make it so each day you log in to the game is different. Randomized themepark events and sandbox events help allow for this.
The solutions
Either add downscaling(like Guild Wars 2), or add some form of purpose to old Zones. Archeage does this very well. Each zone acts as a spot for players to own land, has tactical value for trade packs, and has special weather conditions. Black Desert does it well too, with the ability to capture trade nodes or own a castle, a dynamic weather system, resource system, and economy in which you must transport items between cities.
With features that add purpose to old zones, games suddenly feel a lot more massive, alive, and exciting.
Add Ways To Progress Characters Besides Gear To Avoid Power Creep
One way that PVE content loses its meaning is power creep-each patch when a new raid comes out, old gear becomes useless, and encounters become trivial.
A solution to this is to create forms of character progression that are more subtle and don't devalue old content. GW2 might nail this with Heart of Thorns and the mastery system, but we don't know yet.
Ideally, you provide a reason to run all available instance encounters other than just for gear. The best way would be a daily gold reward that increases in value the longer it has been since you ran the dungeon, increasing up to 300% after a week. That way the "inefficient" dungeons don't get ignored in favor of just running the easiest ones each day. Another way is having unique crafting mats drop in specific dungeons.
Mix Themepark and Sandbox
Both models have their perks. Themepark allows for high quality PVE encounters, and sandbox allows for dynamic content that does not get repetitive. Ideally, traditional dungeons and quests would be placed on a large, seamless world, with sandbox elements being built around it.
By combining the two, you get the best of both worlds.