Hey, it's Kat.
Though this would be very hard to program, this would be great use for places like malls, or apartment buildings. Though you could just sue the renter if he built over his/her property border, it would be more convenient to be able to have a border on a Y-Axis, since making plots in Towny can sometimes be time-consuming and boring, but apartment buildings can use only 3-5 plots at most. Without it apartment buildings are close to impossible in Towny. It would also make good use for more poor players, since taxes would usually be lower because of the confined space.
Again, I don't want to put a lot of pressure on the coders, but this is a valid suggestion in my opinion. Without border claims on a Y-Axis, people will get into fights because of suing and banning in some cases.
Yours,
~Kat