What I think we all can agree on is that WoW needs content outside of the loop of Outdoor Content / Instanced PVE or PVP / repeat, especially if they are intending on keeping the "play the patch" mentality that the game has had since its inception. I think one of the most excellent features that offered additional content outside the Loop was transmogrification. It offered a near endless supply of things to collect, some items (with the best models) are only offered in new content, which means you must group up with players to complete it (I'm thinking like mythic Legion raids).
What I suggest would be player housing. It is something every major MMORPG has now save WoW, and I don't think it's as far fetched as some may think.
For those monogamers who don't often play other MMOs, player housing is where your character gets a plot of land where they can place a prebuilt house (of many varieties) and fill its interior and exterior (within their plot) with decorations, furniture, helpful items, etc. This would bring a whole world of replayable content they can add things to in the future without having to build on top of the existing system they would need to implement to make it work. It could bring some much-needed revitalization to professions and perhaps even forge a new one.
Professions like Leatherworking can make furniture that is fur/hide themed, like those doormats made of the bear pelts you see often in Nelf homes. Blacksmiths can make sigils of the Alliance or of the player's race out of metal, you can hang it on the wall and such. Alchemists can make alchemy tables or perhaps bars where you can serve drinks at. There's ideas for each and every profession.
As for an entirely new profession, introduce Carpentry, allowing you to make furniture like sofas, beds, dressers, etc. A metric fkton of these models already exist in the game and have been updated recently. This would be low stress endeavor for the art team (unless they decided to bless us with a ton of new furniture, but hey, I'm doubtful). However, I am not an artist nor am I a game dev. I don't truly, honestly know how taxing this would be for Blizzard to implement.
As for how the whole system would work, it'd be a lot like how garrisons worked, just with the addition of a UI built for placement/movement of in-game art assets. Everyone will have the option to buy an instanced plot (perhaps larger plots cost more gold?) and you'll have to buy a "prebuilt" house to fill. You will collect art assets as "furniture" (preferably in a separate bag space/UI) and will be able to place said furniture in the spaces in your plot/inside your house.
Finally, the last bit. Potential issues with the idea (except for how taxing it may be for Blizzard to implement). I honestly can't think of much, other than the issue most had with garrisons: everybody was in their garrison and not out in the actual game world. I would wager this is hardly an issue now considering that most people aren't out in the game world, they are playing patch content or in solely the new expansion areas. I would also say, a way to alleviate this is further is to not allow for profession use inside your home plot. No anvils, disable leatherworking, whatever. You are forced to use the cities as a way to use professions, as well as access banking and auctions. Perhaps no mailbox either (might be too harsh).
Pros:
Replayable non-combat content that all players can enjoy in some way.
Easy additions after building the structure for the system. New art can be added as new furniture items in the future.
Potential revitalization of some professions
Potential new profession
A whole new sector for the economy to enter
(I am biased here) Would be awesome for roleplayers.
Cons:
Would take people out of cities, making the world feel a bit less populated, though I address how little of an issue this is above.
I can't think of any more.