Limited content should be for active players. The system need not be strict or punishing. Simply weed out those who quit the game for good and leave the houses permanently unobtainable (many early wards are of FCs that have long since quit for good).

You could go the route of Ultima Online and allow friends to refresh your house while you're gone. 90 days, just have any friend touch your house... the time limit is restored. Make it impossible to lose a house so long as you're subscribed, as well. This makes it so that it's impossible to lose as an active player (or someone who wants to travel), and pretty much impossible to lose if want to take a break with the intent of coming back next expansion, as you have to do nothing but have friends or Free Company mates. It also encourages being social. If one can't do as much as nothing to maintain a house -- and they're not even subscribed -- then maybe they don't deserve a permanent plot when others are willing to at least do that.