My thoughts - one house design per player. Initially you decide on the type of "plot" to build it on. Accessed via phased off-map zone. To access, speak to carriage driver at various marked map locations.
Menu:
My Homestead
&--#60;Guild A&--#62; village
&--#60;Guild B&--#62; village
&--#60;Guild C&--#62; village
The single house design is "shared" across each location, giving each player a single set of storage options, within their chosen design of house.
Plot dimension constraints to be used.
For own homestead, choose setting / external facilities too.
For guilds, a number of player housing slots shoudl be made available within the guild village. The leader can assign / auto assign free plots to players, which will present that player's house on that plot. On leaving the guild you will still be able to access your house etc.
Within the guild village locations, you will see all other player housing that has been assigned, and any guild facilities / shops etc.
Non guild players might be invited to explore or enter the guild village etc.
On logging out or departing, players will be teleported to the caravan location used to enter.
Player housing will NOT be lootable via lockpicking / theft.
House design- modular design will most likely simplify it sufficiently - as per "Heathfire" pack, but would suggest adding a choice of styles ie multiple hallways and main buildings, and further choice of towers / externsions to create signficantly more variety.