I believe that it would fill a much needed niche for there to be a few guild houses, fortifications, and other quarters that are also dungeons, that can be conquered by any who hold them (Subject to quarter rules still though).
The way these would differ from normal quarters is that they would be in fairly remote areas, and have NPC defenders, which will attack anyone that doesn't have a quarter key or belong to a designated faction, alongside an NPC that specifically wanders around resurrecting other NPCs (until she is also slain, resuming if she's raised)
Additionally, they can be conquered by reaching the throne room and interacting with the quarter sign and paying a nominal fee (to represent new heraldry and mercenaries to guard the place).
The change of ownership process should take at least 24 hours and should be canceled by having anyone with access to the locks interacting with the sign (perhaps with a long timer), so to actually claim one of these fortresses, you must hold them well enough to prevent anyone from sneaking in.
These would provide terrain for the various factions of the island to battle over that have minimal lore consequences for the battling. They could also be seeded with rare deposits or a runic chest that the owners are permitted to simply waltz in and claim every 24 hours as normal.
There could be large fortresses, one in the Skalfjord mountains, one at sea, one in the Abyss, or in the hells, and in the underdark, And there could be smaller quarters in various out of the way places, strongholds and supply caches.
There is obviously a fair amount of difficulty involved in this concept, but I think it would be well worth the investment in enriching the gameplay experience, as well as an interesting option for obtaining a quarter