1) Add weekly taxes. Not too much one person couldn't afford for.
2) Add an exponential increase in the taxes accordingly to the number of plots you own on your ACCOUNT. Make it something like: (numbers here are purely fictional, but you will get the feeling)
1 plot/week = 100,000 gil
2 plot/week = 1,000,000 gil
3 plot/week = 100,000,000 gil
and so on...
3) People pay taxes on day 7 of the week. They have until day 14 to pay. If not paid until day 14, house is demolished and plot is free.

4) Watch a LOT of plots go free after 2 weeks.

5) People who actively play get happy.

6)

7) Profit.