From a lore/RP perspective, it'd be nice to have a set of flavor spells that don't really do much for raiding/pvp. They have those pop out menu things now, for portals/pets/etc. They could possibly add some old spells back for fun/flavor using a pop out for them. But mechanically, the game doesn't feel like it is lacking. Classes (at least the 5 or so I play) feel good and fleshed out. I've been particularly enjoying Enhancement Shaman, Balance Druid, Arms Warrior, Frost Death Knight, Frost Mage, Beast Master Hunter, Assassination Rogue, and Retribution Paladin.
Glyphs are a cool way to add a layer of flavor to your class, and I think the game would benefit from expanding upon them. Slowly add a few options per major spell or something.
The old talent system was bleh compared to MoP and beyond. I suppose they could combine the systems, but cookie cutter was standard.
Professions do feel unsatisfying nowadays. I wish they would bring back some of the layers of cool items and fun consumables. Alchemy was fun at the beginning due to all the weird situational potions available, but those got dull after a patch or two. The biggest issue with professions is letting old stuff become unusable.