I don't think the expansion needs a bunch of new features, but it needs to really rock the features it has. For the most part DF feels that way.
The main issue i have is the talents trees, and UI update are system updates. Even if you don't buy the expansion, you will get these features. To be frank these updates are core system updates which should be support by our sub fee. On top of that the wider pool of M+ dungeons are just better implementation of existing content.
Also, Blizz is incredibly notorious for not following up on its features/products and leaving them to die. If they do nothing to merge flying and dragonflying, then we'll have a big issue come next expansion when they need to decide if they'll make all our old mounts or the 5 dragonflying mounts obsolete. I'm expecting another MoP->WoD, Legion->BFA kinda of scenario where we'll have great bones, but they'll just tank it because of poor follow through.