A simple thing to do would be to put in some kind of housing tax that has to be paid every week for upkeep of a house.Set at the right rate it could potentially cripple house hoarders and force them to relinquish some of their property due to upkeep.
Also you could have the houses relinquished if after a certain period of time payments were being made.Make it a month or so.This could free up all the properties for those who never log on but still own property.