What if you took all the current house/guild house interior levels, duplicated them, and added more doors or stairs to the current layout for the duplicate level for more rooms. So what you'd start out with is the original house/guild house, but you'd have the option to upgrade to the duplicate level for an extra $# of gralats to gain access to more rooms to what you currently have. I can see this working better for guild houses, because with houses the most you'd probably get is an additional room, nothing else really because houses are meant to be small. Guild houses should have up to 3 new rooms, just because they're so big.