I would even like to see planeshift go beyond guild houses and towards guild settlements and cities.Any guild could have control over a city of their own. Allied guilds could share a city and of course the richer or stronger the guild the bigger the city can be. Also guild member's spawns could be moved to their cities. The NPC's in cities would also be more kind and generous towards their rulers. Guild members get discounts on wares and have an easier time acquiring information or quests from NPC's in the city they control.
I'm thinking of a few different ways to obtain a city ... the first being building their own, something which has already been done in many games. Your guild just finds an empty space and starts building their own town there.
Other options would be to acquire political influences, come to work under a vigesimi or maybe even an octarch who would be in charge of cities like Hydlaa or Ojaveda. Of course this position isn't permanent and while one person may be in charge they would have to be backed by one or more influential guilds to stay in power. The guild would then pick up certain aspects like providing the city guard, supplies, owning stores and renting out houses or workspaces, running the tavern, etc. The influence needed could be acquired through roleplaying or questing, doing powerful people many favors. Mainly I'm guessing the GM's would integrate shifts of power into events they'll host though.
A third option could be invading and taking a city over with force. If you have a guild full of fighters and don't want to spend the money building your own city, you just let another guild build one for you which you then happily take off their hands with the optional bloodshed. Of course this option should be the hardest to pull off and actually require sieges. When a guild owned city is being targeted for attack, both guilds need to declare war with eachother first. The defending guild will also enjoy some benefits as long as they stay in their town. The towns will most likely have defensive structures like walls, towers, anti-siege equipment, ... on top of that defending guilds should get a number (depending on the guild's membersize) of NPC troops to defend their town when there aren't (m)any guild members online.
The attacking side could attempt to 'starve/bore' them out by simply waiting for any members that come out to kill them off until they give their town up or break the siege by storming out in full force. Of course their defensive benefits will be lost when they leave their city ground. There should also be siege engines, projectile launchers, climbing devices, siege-magic maybe even and more available to the attacking guild if they can afford them.
Lastly I think having a rule that no guild can be in (partial) control of more than one city at a time would be good. It'd prevent powerful guilds from controlling all cities and leaving newer guilds no chance and it would prevent guilds from taking over city after city by force. When you already have a city you can conquer another if you like but then your old city goes to the opposing guild by default, unless they refuse it then it can be claimed by whichever guild claims it first.
It would be an interesting aspect to bring politics and rulership into play ... but difficult to balance and keep in check maybe.