I saw two style types mentioned, one royal feeling like Qeynos and one gothic architecture like Freeport.
From this I'll move on....Have a live event to have them made room for and be built. The orcs and fight the encroachment of freeport into the commonlands and the gnolls can fight qeynos...Have us work for it or some kinda lore on why the factions of qeynos or freeport are pushing for the guilds to have a seperate outpost...
Have the actual building be in an open instance like an overland zone and the entire area around the building can be traversed so that it gives a feeling of grandure. You(SloE) showed signs of this with Unrest, I feel that if you would have given us access to the back of it and the back of other large castles like mistmoore you could have used the space to do alot more and give a larger feeling of depth....
Ok, now here is another idea that I like alot of people will agree with....
Why not make this INSIDE the castles of both cities?? Open up the castles and high enough level guilds can inhabit diffrent large areas or wings of the castle because they have grown to have such prestiege in their city?(If your thinking this will prevent faction sharing guild halls on PVE then just make the area of the castle instanced and when you click the instance it gives you the option to enter your guild or visit, and when you do you can enter one here or in city of your guild, when you exit it will send you back to the main castle of your home city.
Making something grand scale like that will really add to the depth and fun of the game.
Room Types
------------------
Practice Area- Select from Scout, Mage, Fighter, or Priest mob. Then select con. type. Anything from triple down solo to x2. Having a x3 or x4 is a bit much. (With a x2 have a list of diffrent mob tactics. Example: Mem wipe, AoE types, random teleports, hate list shuffeling, etc etc)
Tradeskill Room- Preferably 2 max of each station type, writ giver and a fuel/supplies seller. Fuel/supplies seller is free, but the stations cost to intially buy for each one.
Same room you can have a broker that also allows for you to sell at a large discount or for free to anyone with your guild's tag, along with normal broker abilities.
Meeting room- Sitting areas and a large enough stage to stand infront of(If a real meeting is going on then leaders can just stand infront for the sake of effect, other then RP it won't serve much use for other guilds except maybe a focus of attention...)
Tropy room sounds good, but minature models and heads are kinda creepy....Maybe just have a large crest of mistmoore w/ a particle effect if you clear MMIS, an item that has a small scale effect of the moon blowing up hologram from NTTR, some nature type thing for clearing EH....dunno but something that symbolizes the zone not the mob is a better idea IMO...
You know those mini spires that have two arches that you use to travel from one KoS zone to another?(Has a faded image of a part of the zone your going to in the center) Make those a expensive crafted item(several rares, and transmuted mana from t7) from recipe that drops in vaults of al'erad(spelling?)...
Limit the items that can be placed in your guild hall....Your fucking insane if you really like having pets that move around and get in your way all the time.
To those who think that this will take away from the community then remeber that we can already just get a home really close to a broker and put crafting stations in our homes and craft...
I was really bored when I wrote this..../sigh