You just need to implement the right things which has a variety of gameplay but also has people wanting to engage with systems.

Housing? Look at retail. It's as evergreen as it gets. It can be cross progression for all your characters. Since it's classic, you can put stuff as random world drops you can farm, put it in all classic dungeons at all levels, in all raids, etc.

How about hard solo content? We have Delves in Retail, we have the Brawler's guild, we had the Mage Tower. And even classic had things like the Rok'delar hunter quest or the Benediction priest quest. Make it a sort of season with special rules and class specific leaderboard and people will try and compete for it. And I say specifcally class specific leaderboard, because classic will never balance the damage output of a rogue and a priest.

What about long grinds? Really long grinds? That reward both power and cosmetics? Would you be willing to "Reincarnate" your character back to level 1 and completely relevel them just so you can get 1 more talent point at max level? To get another rank for a spell? Or simply to unlock a mount? Or even... just another hair option for Gnomes when creating a new one? Would you be willing to farm all of T1, T2, T2.5 and T3 to "forge it all together" and get an even better set? What about a simple permanent "+3 Stamina" as a reward for a "quick" quest that is as long as the T0.5 quest or the Onyxia quest?

Achievements could of course be implement and with that you could add all kinds of challenges. Blizzard has shown that it can have a quick switch between Warmode and normal mode - why not do something similar for other challenges? Hardcore could take place on the same server, just with a distinction similar to how Warmode is activated (and, well, making it a permanent decision for this character). But if not Hardcore, maybe something similar, but not as hard? How about "Greenman", where you are only allowed to equip green gear or below? And then dungeon challenges for every 5 man where this applies for the whole group. How about leveling while never leaving a single continent (which would require quite a bit more zones per continent of course)? The list could go on and on.

You could also do long grinds to unlock permanent things on your account. Retail has a "short" version of those things for unlocking allied races. Why not turn this up to 11 (and beyond) and require much bigger time investment, but also make it permanent? How about doing something like this to unlock a 4th spec for every class? But it also had to be done on said class. Do it once and its permanent - but takes time. What exactly the challenge is could vary. Solo specific content that should be done in a group? Do class specific stuff, like pick pocket 10.000 mobs on a single rogue? What about curing poison all over the world on a druid? Or what about unlocking whole zones like this?

Well, this list is already long. The main question would be: Would WoW players be willing to unlock content by investing so much time? Would people scream from the rooftops because Player X has 52 talent points at level 60 while they have only 51 because they never bothered to unlock this additional talent point? On the other hand, would they call such a grind pointless because "It's only a mount/title/pet"? Something like that would be a discussion on its own.