I feel like there should be some sort of incentive to go from having a house to not having a house. As it stands, there are plenty of players that own a house, but don't really do much with it. But since they already have a house, there is no reason for them to give up the plot since there is no downside to having an unused house once already owned.

Having some sort of overtime cost to the house would provide some incentive for people to give up plots that aren't being used.

Another solution (that doesn't directly involve adding more plots), would be to increase the permission system for housing. So that a group of friends can share a single house with everyone having full permissions to modify and use everything in (and outside) the house. There are several instances where friends want to use the same house (exclusively between them), but don't want to be in thier own free company purely for that reason.