Good question, Ken. Here are a few suggestions:
-You could put a special badge on squares owned by current buyers in a certain part of the site so that its easy to see which cells are 'special'
-You could charge a small fee for auctions on squares, and make "special squares" fee exempt
-You could take a portion of the ETH raised on each future square, split it up and send it to the current holders of the "special squares".
-You could allow "special squares" to upload animated gifs
-You could extend the color palette accessible to "special squares"