You want housing-related gil sinks? Here's a couple ideas:

1) Let FC members buy more than one personal chamber. Each FC house can accommodate over 500, and most FCs aren't that large. FC leaders could set limits on number of chambers allowed by rank. Have costs escalate the more you buy. Maybe keep the first one 300,000, then 600,000 for the next, then 1,000,000 and go from there.

2) Turn on personal chamber functionality for private estates, so private estate owners could expand their space without having to necessarily buy a larger house. Same escalating cost structure. Fat cats could build 100 room palaces in small plots.

These ideas would have the added benefit of stimulating the furniture market, and the associated MB taxes would pull even more gil out of the economy.