If this was Vanilla and you asked the same question, I would have suggested:
Pet collecting/battling since pokemon is a fun game
Wardrobe features. So many cool looks I want to make outfits.
Some sort of optional story based character progression. I want to play a story, but at my own pace! (Covenants)
Reason to go back into dungeons after the initial leveling process.
A in-game card game or chess like mini-game like some JRPGs (order table)
Now that they added all that, the only additional feature which I could see them adding is player housing. Again though, it's a resource vs time thing. The more alternate things they add to the game..then the more they have to maintain.
I honestly can't think of anything else I'd want to fill my time
Maybe some sort of like pvp/mythic+ combo mode where you race another team through the same instance...oh wait they tried that with islands...damn nvm.