This is a suggestion for a small, four quest Adventure Pack titled "The Tower House".
In this questline, your party would be tasked with clearing out a Tower House in a remote area over the course of four quests. The monsters infesting the tower house (and the surrounding area) would correspond to whichever favor faction it was felt could use four more quests the most. After completing the questline, the player would take control of the tower house. The main reward for the questline would be a 25 slot trunk in the main room that functions similarly to a guild chest, but could only be accessed by the player who could use it to store bound and unbound item similarly to the bank.