When people sculpt carcassone into 3d reality its wonderfull.Closest thing I've seen was the Xbox Live Arcade version where the gameplay is identical, but the features were animated, played sound fx, and "popped up" from the board when completed.
There's only one variant I've never seen anyone implement this 3d medium to its fullest.
Basicly they would be treated as though a unique "city" set.
Distinct edges and grassy areas would highly outnumber them.
A few would have roads that go over them as 'passes', mayby -a- cloister.
On the whole just another "city" set alternative to spice things up.
Any more than that would in my believe over complicate it as an expansion.
What 'expansions' would you love to see in 3-d?