Vanilla: Fight old God, save azeroth. Then stop the servant of the Lich King, Kel’thuzad and save Azeroth. Or spam nax till you have full t3 set then stop playing.
TBC: Defeat the burning legion by going to outland and defeating the emissary of the burning legion, Illidan, and save Azeroth. Or spam Hyjal and BT for Tier 6, stop playing for a while, then get your butt kicked in the Sunwell and cry to blizzard to nerf it.
WOTLK: Defeat old God, defeat the lich king, and spam ICC for more than a year.
Cata: Hyjal is on fire, go kill Ragnaros and save Azeroth. Deathwing must be stopped, we need you at deepholm to save Azeroth. BUT WAIT, there is more, twilight’s hammer is trying to use forge of origination to destroy Azeroth, go to uldum and save azeroth. Dailies, dailies, and more dailies. Or do dragon soul till your eyes bleed and cry in the forum about how much dragon soul sucks and how much you hate revamped old world.
MOP: Both faction finds a new land, then recruit the locals to join your faction, OH NOES we have to stop Garrosh and bring him to justice. Or collect all challenge mode transmogs till next expansion.
WOD: Sounds of cricket and a tumbleweed rolls by.
Legion: Stop Burning Legion 2: Electric Boogaloo. Or spam mythic+ to make your character as broken as possible, and you can also keep collecting mage tower transmogs till next expansion.
BFA: Heal azeroth’s woooons. Recruit allied races for the war against the other faction in the warfront, but it’s really just AI. Sylvannas kills Saurfang while twirling her moustache and then it is finally time to defeat yet another old God. Either that or the end game is to collect your best corruptions and make your character as broken as possible.