Tax properties via total area they occupy. This would limit the 600 x 600 chunks that are simply claimed but unused. Obviously it shouldn't be a huge tax, but it should cost you money to hoard land. And yes it's hoarding even if you "plan" on using it someday.
Lastly I'm not void of this criticism, I have a desert claim that is larger than necessary and I still believe that this would be a good implementation. Just my 2 cents. I'm fine with no tax, but damn is there a ton of claimed land that isn't being used.