But to look back at WoW, what could they do that would bring their players back and make the MMO interesting again? Well, sadly, I think it would take a roughly 180 degree turn from what the Mists of Pandaria expansion was. They would have to address the "grindy" nature of the expansion, but at the same time expand what's already interesting and fun.
No new classes or races
I don't think adding a new class or yet another race is going to improve WoW. A new class will either repeat the pattern of other classes (as the Monk did) or will create a massive imbalance in the player base as everyone scrambles to play the new hotness. A new race can't really change what's wrong with the experience.
Instead, I suggest adding a new spec choice to each existing class, and in the process, introduce something I've advocated for a long time as a solution to the scarcity of tanks: the tactical support role. To break that down:
Monk, Hunter, Warrior, Mage should all get a tactical support spec (for Hunter, I think it should be their existing Survivalist spec, and a new spec should be added that does something new like a true melee spec). This role should allow these classes to perform complex pulls and generally support the damage, healing and survivability of the other party/raid members. To compensate, new dungeons and raids should be structured to require a tactical support characters abilities in order to pull groups of mobs or initiate boss fights. What this does is to break up the traditional role of the tank into two parts: the "leader" and the "soaker". The tactical support becomes the leader of the group, deciding when and how to pull. The tank takes on the role of soaking up damage and managing mobs during combat. There should be insta-death implications to not using tactical support (e.g. having two mobs that must be separated on the pull using tactical support abilities (not CC) which will wipe any gear-level group should the be pulled together). This change doesn't help if overgeared groups just ignore the tactical support role and demand that the player in that role "go dps".
For raids, tactical support should be "shaman on steroids". They should amplify the abilities of the rest of the raid, but this should not be passive. They should get a suite of cooldowns that allow them to choose when and how to enhance the raid. A cooldown might enhance a priests shields and allow the tank to survive an otherwise insta-death mechanic or it might provide a larger area for all AoE spells during an add phase. Just standing there and casting buffs isn't interesting. Being a tactical support role should mean that you have to think about the tactics of the fight.
As for the other new specs, I think we can see where most classes are going, but I'd upset the trend a bit: give Warlocks a healing spec themed around dark magic (your tank might be at full health, but he's feeling "unclean" about it...) I'd also throw a ranged spec at Death Knights (perhaps mount-themed? mounts in combat would be an interesting addition).
Reduce the grind
Don't continue the farmville foolishness. Seriously, whoever thought adding farmville to WoW was a keen idea should be moved into the basement without a stapler! Beyond that, I think the golden rule needs to be that it should never make sense to spend more time grinding than raiding. Raids should be hard, especially on heroic, but their difficulty shouldn't be a matter of needing lots of mats and rep that players grind out doing endless daily quests.
Introduce "fun" guild activities
Take a page from Rift's expansion. The Hunt Rifts were a huge hit, drawing guild and random groups alike to bang out these fun events. Yes, there was a rep grind behind it, but the events themselves were fun. If WoW could introduce something that was both casual and required moving around the world (no queuing!) I think that would help quite a lot.
For those who are unware, Hunt Rifts were an open-world event. Players get tokens from normal quests and zone events (something else that might spice up WoW) and spend them on buying triggers for these events. They then go to random spawn points for "tears" in max-level zones where the trigger starts a localized event that scales based on the number of people present. It's a simple event in which waves of invaders culminate in a boss that's also scaled to the number of players present. Whoever opened the tear (and their group) are eligible for a bonus chest at the end, but everyone else gets rep and an achievement (if that was the first time they beat that event, of course). Once you get all of the achievements for a tier of Hunt Rift, you can buy the triggers for the next tier which has better rewards in the bonus chest.
Use the world
I can't stress enough the value of WoW's real estate. The world is huge. Use it! Cataclysm did this mostly right, but once players were max-level, they never went back to Vashj'ir except to grab the hunter pet.
Don't wimp out. Add a full 10 levels and bring the players up to 100. Yes, you need to re-tune the game for level 100+. Do it. Five levels of progression, no matter how long it takes, just doesn't feel right.
Also, I would slow the rate that new players level down to vanilla's speed and remove all expansion content from the leveling path except for the new expac. Instead, have level 60 players undergo a special solo event where they gain all of the abilities of levels 60 through 90 in a way that forces them to use everything they've got. Imagine a phased event where each phase grants 10 levels. Learn to use a couple new abilities, ding 70; a couple more, ding 80; the last two; ding 90.
This does several thing: it brings back the feeling that those first 60 levels are important, and not just a fast-forward blur of ding-effects. It also removes the strange sensation that each expansion's leveling content gives which is that there's this building and interesting storyline that's going to lead to something... but never does. Expac leveling is part of a story. If you're not going to have players experience the rest of that story, don't make them play through it. The other nice benefit of this is that expac quests are still there, and players can do them for some gold at max level or for the story line, if they wish.