Would you like it if Warcraft done same thing as FFXIV?
Basically, to enter new expansion area, you need to complete all former expansion main quest, dungeon and raid.
For example, As alliance, You need to clear first zone, see Velen death, see Akuma death, see Garrosh death, complete all raids at least once......etc
You can add some side "main quest" like seeing how Yrel change in every zone, from weak character till she become strong character.
You enter Legion while knowing what hell is going on, because you saw everything, from saving Gul'dan at the beginning till the end where Archimonde shot him to main universe.
-New players need to complete past content first to see newer one.
They can make it easier with account achievement, where you only need to do all that shit with one character.