That’s a tough one. It kinda depends on what counts as customization. If dances count, it would be really nice to be able to choose new dances. At the very least, it would be nice if it were like WildStar with a /dance1 and /dance2. Dance2 in WoW could just be a faction dance that all members of that faction, male and female sorry body type 1, 2, 3, and 4 can do. That would only mean making two new dances, Blizz. Though it would take some tweaking for each race.
After dancing, I’d say just make sure all the races, and I mean all the races, allied too, have close to equal amounts customization options. It’s pretty sad that come races have like four pages of customization and other races have one.
After that, mechagnomes need love. Mechagnome Customization Thread
Dracthyr need access to mogging in dragon form.