Here are some possibilities, none are really anything more then that.
Guild offices: a place in a city or major town that is owned by the guild?can have a few small things including a guild bank.
Guild forts: areas fortified and built by a guild or bought by a guild?depending on where it is settled would also possibly make it liable to taxation. What can be there???
Trophies and any and all tools necessary for the many crafting abilities in the game. A guild will be able to upgrade and modify to a degree?I hope that none of the layouts are the same, even though many parts may be.
Guild Lodges: the middle point, a large building usually in the outskirts of a city or in a small town.
If we allow any type of rapid transit system, Lodges and forts will be made destinations (only for members of the guild), cities already will be. And they will also function as bind points for the guilds.
Will other groups or creatures destroy our buildings?(NO) but they may make the door pointless if you do not hire guards. All things will require upkeep and maintaining this will be expensive, (as expensive as buying it or building it) but that?s what guild dues are for, and the more focused the guild the less you have to maintain.
There hopefully will be guild tasks that can be done instead of paying for maintainers. But it will not be something that guilds can take their properties for granted. They will serve a central role for their guild, and there may even be a time where Real Estate is a business, not because lack of land, but because lack of choice properties.
There will not be forts within eye sight of each other?lodges are a different story and offices are very much a different story.
In the end I expect every guild to have an office (except the stealth guilds) but few to maintain more then one fort.
Castles and such will be really really expensive but I hope that there are many in strategic locations.
If done right (a guild might make a fort near a dungeon) and can buy loot from dungeon dwellers, those can then be sold in the cities at a large profit, or used by the guild.
In the end we will see.
Guild offices, Lodges, Forts are the three ideas I have?a fort being anything from a building with a large wall to a large castle. If you have any other ideas (toll bridges) I am willing to read.