I like the theme. I don't see anything wrong with limiting the buildings to being built in Bryce, but some of the extras that dress up a scene, like people, animals, plants, curtains (I never could get the hang of making curtains out of the terrain editor) and fountains etc. might be a bit too hard to make all in Bryce. My 2 1/2 cents worth.
But if Nazul says anything goes then I don't have a problem with that either, it is up to the winner to call the rules :)