If a player leaves the game, leave the housing intact. The housing should go up for rent by the Empire. Players can 'rent' one of these if they want. Unfinished plots should stay for a while and be available for purchase as a "fixer-upper", only to come up for "salvage" after a few months. Salvagable houses can be deconstructed to novians by crafters, and the exact time will not be predictable. If a player purchases a prebuilt abandoned house, they should get the option of transfering it to another plot.
A ghost town full of houses looks better than the current wasteland.