I wish Lego made a set that made it possible to do a corbelled dome, a la the Mycenaeans -- although the blocks would have a different size with each row, making it a one-use set.
Of course, people have accomplished so much using the standard available parts:
http://www.andrewlipson.com/escher/ascending.html
http://www.andrewlipson.com/escher/relativity.html