The Sha was really cool, and while I miss them, their decline did feel natural seeing how they were tied to the old god heart and negative emotions etc.
The scourge no longer being a massive world ending threat as it was, has also felt natural but they were probably one of the best villian factions. But Bolvar is seeming to be up to no good, so we might get a return.
Legion was a massive blunder, they were a good threat, but throughout the expansion named after them they ended up feeling like less of a problem than the Defias or murlocs. Just a mundane annoyance rather than the superb evil it had been hyped up to be. And that is a shame.
Twillightâs hammer was also great, but like Legion, their finale/end felt sloppy, since just killing Deathwing somehow automatically stopped their coordinated assaults across Azeroth, because reasons. That felt weird.