Legion, the newest WOW expansion, does something that the game really has never done particularly well before - reusing content. It used to be that you killed some mobs in Westfall, finished the zone, and then basically never went back unless you really wanted some obscure achievement. In doing so you also levelled past multiple other zones at the same level so there was always a huge amount of content that you never really got to experience unless you played multiple characters.
In essence the things that Blizzard created were usually really limited in use. It is far simpler to create content this way because it requires little planning and design can be clunky and inelegant. However, you spend a lot of effort making big zones that see limited use. Sloppy, but simple.
Legion is quite different. Blizzard is working really hard on making the content reusable to a large degree. One way is the way that the zones all scale with your level so that max level players have a large area they can usefully be in. Blizzard also added tons of world quests with important rewards so players spend a lot of time running into each other on the map instead of chilling in town or doing instances. This is all really quite good, to my mind, and definitely lets them get a lot of content in without building more areas.
The other thing they have become good at is stuffing more things into a space by using three dimensions. Legion is chock full of caves and cliffs and reasons to go up and down. In Suramar city they level this up even more by having lots of encounters on ledges and rooftops, and there is plenty of hopping from beam to beam to trying to collect treasures and mana bits. You run around the city a bunch, then eventually realize there is another whole level above you that you need to pay attention to.
It must have been a pain in the butt designing all of that! Figuring out which things are part of quests and which things are also being used for end game content and fitting it all into the world is a challenge. So many puzzles pieces going together means you really need good communication on the design team.
It feels a lot like the way I was taught to code back in the day. You can build a new piece of code for every situation, but it is generally far better to build it in reusable pieces instead. That way you can be a lot more efficient overall. It requires you to plan ahead more but in the long run building reusable code saves you a huge amount of time and is so much easier to work with.
Building worlds is complicated. I think Blizzard has taken a lot of their lessons from the old days and really put them to good use, and it shows.