Upgradeable Portal Gate/Device which requires you to complete all quests in any zone to be able to teleport there (has cooldown)
Backyard Stable which puts one of your mounts on display and grants you upgradeable mount speed in the zone your house is built in
Armor stand for transmog
Library upgraded with archeology or new independent profession by exploring existing and old lore in the world. Unlocks cinematics and chapters from warcraft novels ingame in the safety of your home
Outdoor and indoor skins and appearances (flooring, rooftop, wall types etc) unlocked from reputation gains with factions and your characters race.
Fireplace for cooking
Screenshot picture frames
Your favourite pets wander the housing area
Wardrobe acting as transmog set changer (limited in use to avoid making city transmog npcs irrelevant)
And more stuff unlocked via guild achievements and contributions (guild levels revamped to focus on housing accessories and decorations)
I know instancing would be required for such feature to be part of wow but I also thought of making each guild have shared housing plots instance connecting the guild members. That would make a new problem when two members occupy the same plot but would definitely make the world feel bigger when you can take a visit to your gm in the wicked forests of duskwood or in the molten depths of blackrock mountain