My solution would be as follows

1. For those that own multiple plots grandfather them in, but they cannot relocate or buy another plot till they are down to the set limit.

2. Offer an amnesty period where those that do own multiple plots that are over the set limit can relinquish them for A full in game market value refund until they are at or under the plot ownership limit. (limited time say a month)

3. implement fair and reasonable restrictions on how many persistent housing plots that fair and reasonable

4. Add in some sort of functional every man/woman possibly instanced housing/deluxe apartment(condo) that offers enough room to really be creative and offer effective services concerning gardening, engineering and other housing locked content.

5. Allow each character on the account automatic full access to all housing and apartments on that account.