Professions - I think professions should play a role but not huge role in player housing. I think based on your profession you can passively earn resources from killing mobs etc. The resources should just drop from anything in current expansion making it very easy to passively farm them. These are used to craft structures/objects for your player housing.
I think there should just be a few items for each profession you can craft. Some unique items should come from achievements, quests and item drops.
I think after you get a new item or structure it should be “unlocked” and you get more from either a vendor or through crafting.
Place-able Vendors/NPCs - You should be able to unlock NPCs from achievements and some you have by default. These vendors/NPCs should be for Transmog, Profession vendors, Trainers, other general vendors. I don’t really think they should but if they add an auction house, it should be very expensive or very hard to unlock maybe exclusive to a “Guild Hall”.
Interactable Objects - You should be able to freely place things like training dummies anywhere and however many you want. The chairs and everything should be fully functional and interactable.
Guild Housing - Your guild should have its own private neighborhood either separate from your other private neighborhood or it should just be the one private neighborhood you have and it should include a fully customizable Guild hall that is larger than the player houses with additional features that also includes everything you can do with the player houses.
You could add more rooms and sections to the guild hall. One room could have statues that reflects the players’ characters in the guild.
In this Guild/Private neighborhood you should also be able to build other structures like an arena that automatically activates PvP like Gurubashi Arena and should also have some customization freedom with the layout, objects or entire structure. There could even be a dueling room inside the Guild Hall and you can set up a dueling tournament system.
Building Permissions can be changed in the Guild Rank Permissions by the Guild Master.
Please take some inspiration from Warframe’s Clan Dojo. You can add additional rooms etc. and customize every room.
If needed for larger guilds because of the 50 plots maximum for the neighborhoods you could make it where players who are currently in the neighborhood has their house take up a random open plot. So maybe only 50 players can be active in the neighborhood at once. Once you leave the instance your plot is empty and someone else who joins would take up the plot. They even do this in Roblox Tycoon games where you join a server and your saved tycoon takes up a random open plot.
Resources - Resources should be passively farmed from every single form of content, open world mobs, PvP, dungeons, raids, delves. If this is the route that is taken with how you obtain structures. They should definitely be easier to farm as time goes on and you unlock more things. There could even be a contributing system for Guild Neighborhoods where you can contribute resources to a structure or an unlockable place-able item that will unlock for everyone in the guild.
Music - You should be able to set the background music in your personal house and guild hall to different choices, new or old soundtracks.
Stables - This can be outside your personal house and should be able to house at least 10 mounts hopefully more if possible. There needs to be some way to show off your collections. There could possibly be a hunter specific stables as well that shows off some of your hunter pets that walk around. Hunter stables should also be warbound as in all of your hunters on your account should share pets and stables.
Armor Stands - This is a must item you can place in your personal house to interact with and apply one of your saved transmog sets to it. It could have a choice for which one of your characters it reflects and then choose one of that character’s saved transmogs or you’ll just have to log into that character and place it. It could be set as any race or just the race you place it on. You should be able to place multiple of these at least 10.
Dueling - This should be enabled and allowed anywhere outside in the neighborhood.
Free Building outside of housing - In guild neighborhood, guild members should be allowed to freely build outside in the neighborhood so you can make for example a dueling platform, an arena, or just a chill spot like a park with benches/decorations. You could possibly even add in some ponds, bridges etc. Some of these can just be prefabricated structures.
There should either be a dedicated space meant for free building in the neighborhood or just enough empty space to allow it.
These building permissions can be determined by the guild leader as a new setting in guild rank permissions
More organized list of some possible items/structures:
Training Dummies
Arena
“Transmogable” Armor Stands
“Transmogable” Weapon Wall-Mounts/ Weapon Stands
Mount Stables
Walls, Pillars, Foundations, Platforms, decorations for outside building in guild neighborhood