Different levels in regards to terrain would probably look really weird. It is possible from a multi-story terminal perspective, and I can see how it would work, if a taxiway would change between two levels (z-axis for us) in the current 2D perspective it would look as if it would soar over the edges in an impossible contraction.
However, we do break the logic here actually since grass stands connect via a direct way (same level), while asphalt stands with a jetway attached allow for vehicles to pass below just the same… perhaps we’ll just abstract that out.
Anyway, different terrain levels for terminals, no, probably not explicitly at least, matches badly with the 2D perspective and would probably cause confusion.