If I made a WoW expansion, I'd repeat the motto: a WoW player doesn't like killing things, but trying to.
It would increase the difficulty, bring the need to have good survival skills like mana efficiency back, reduce leveling speed, make uncommon+ gear much harder to obtain and herlooms just above common, and expand the current system to be reduce the amount of spells you have and make classes more diverse and useful.
moving on, it would be called 'chronicles of the old gods' and it means that if you complete the main story of a zone and reach the maximum level of that zone, you would be able to unlock a new version of that zone which eventually features fighting a 'new god', which is like one of the old gods, in the story of that zone, and that zone would also have dynamic difficulty, scaling to level. the final boss of the zone is insanely difficult and near impossible to kill without a proper raid group, but at least it shares the loot with all who helped kill it, depending on how well they did. if they kill the boss enough times and well enough, they would combine the currency gained to create the 'heart' of that boss zone. once everyone in your raid has every single one of these boss hearts, they can combine them at the maelstrom to summon the old god form of deathwing. and if you were disappointed by the madness of deathwing encounter, this one actually has you fighting deathwing the way you've always wanted it. besides deathwing actually walking around the maelstrom and being massive, he would crack a pillar four times during the fight. one pillar floods the world, another causes an earthquake, the third one burns up the land, and the final one will magically shatter the earth into pieces. however, if the raid survives to this point, then there will be an event which stops the earth from shattering, and allows the raid to fight the true form of deathwing.
but even though there is a boss for every zone, there will be an extra boss for the night . in the night cycle, there would be a beast called the wanderer, which is deliberately meant to be horrifying. you will not be able to see its' true form, even if it attacks you. if you aren't near a light source or campfire, then the one spawned in that zone will pick up on your scent, and will begin to find you if there is no one else it has smelt. most of its' spells are instant death attacks, and if a raid is gathered, the wanderer is designed so that 3 people will die before its' basic attacks are no longer instant death while it is in combat.. its' true form will only be revealed when it is close to death.
heroic dungeons and raids would be extremely hard to do well at and then complete, but give great rewards.
I have roughly 20 other game mechanics I have theorized and tweaked for WoW, but I'm just a limited access account, what would I know?