I think that it would be nice to be able to adjust a plot so others may claim plots under or over it.
For example when building a road this setting could be used if you dont mind others building a bridge over it or a tunnel under it at a later time. Then the road is guaranteed to stay intact but will enable others to use the rest of the space. It might allow more interesting cities to develop.
Alternately that it is possible for a player to request to build inside someone elses airspace, then the plot owner can choose to allow it or not after reviewing it. Maybe similar options has already been considered.
I think that the current system which reserves space below and above a plot to be great. But it would be nice with some more flexibility.