How about a chest item(like company chest or retainer) that players who own a house and have tenants can split inventory space?
I was also thinking of bond partners only but it's probably a bad idea to restrict it to bonding..
Basically an item with shared slots that will allow the owner and tenants to deposit and withdraw gil, crystals, items, etc instead of having to put them in the FC bank. Maybe 50 slots. If that's too much, maybe 25 slots.