I suggest that players who own a guild be able to spend another set amount of gold (Probably another 1M)
In order to purchase a Guild Hall
You could charge extra Gold/Reales to add things such as decorations and Mob spawns
(Very limited mob spawns if any at all)
(Decorations shouldn't be to hard to code as each guild most likely has some identifying piece of code that would load their specific purchased decor)
I got the idea after doing some thinking about how the crypts work,
Each one acts as a separate map because multiple different parties can do the same crypt at the same time and never see each other.
The halls would work the same way,
A location on the map is decided for the entrance (Most likely in varlone)
When you try to enter that location it detects if you are in a guild and if that guild owns a guild hall
And then takes you to your Guilds Hall.
I'm not sure if castles have some of this functionality or not because I've yet to be in a guild that controls a castle,
But if that is the case it would still be nice for small guilds to be able to get their own place to hang out.