I think that a room where you can store weapons and armour would be really cool.
You could have a locker that stores 1 of each of the boss weapons (Like the costume room), a small shelf for storing the god books, set of racks on each wall for storing staffs, bows, and melee weapons, 3 armour stands in the corners for holding a set of equipment and in the centre a raised dais where you can quick equip a set of the armour and weapons, fantasy Ironman style.
Obviously it'd need a high construction level and I'd say that the quick equip dais should probably also need high magic and smithing.
I just like the idea of a POH being an upgrade to the bank.