Jun Ogawa raised an interesting problem (in successive conference presentations) that there needs to be a way to describe surfaces and zones in three dimensions.
Currently the points attribute allows 3 to infinity teidata.point values. Although I think it may need more fully specifying, he and I both came up with the idea of adding an optional third coordinate to the X,Y coordinates of teidata.point to enable the Z axis to be recorded.

Currently the content model of teidata.point is:

 <dataRef name="token" restriction="(-?[0-9]+(\.[0-9]+)?,-?[0-9]+(\.[0-9]+)?)"/>

which shouldn't be too hard to change to have an optional third point.