W. Vostinak,
a good and interesting post.
To continue the jazz analogy, yes, Coltrane's "Ballads" are a lot different than his "Giant Steps," but yet I'd say that both are instantly recognizable as Coltrane.
Is that the same/similar situation for the great architects you mentioned? What I mean is, did they have a bedrock "philosophy" that allowed them to produce quite a bit of variety in their work, yet always be themselves?
I think that's what I was getting at, i.e. that C&C have the kind of bedrock philosophy that makes them instantly identifiable (like Armstrong is on the trumpet), but that in their case the differences/variety comes about, as Bill McBride suggests, because the properties/land they work on are all a bit different (sort of like Van Gogh painting a flower one time and a field of flowers the next, but always with completely the same style).
As was said a while back by someone who knows a lot, this could all be true, and you can absolutely love their work, and yet you could still wish that occasionally C&C were more "bold" in their choices. I understand that sentiment. I just think it might be a little bit like asking Louis Armstrong to play be-bop: he could do it, and the results would be interesting, and you'd probably still know it was Armstrong right away, but that isn't really what he wanted to do, and there were a lot of other fine musicians already playing bop.
Anyway, just thinking out loud, and asking questions. And your main question remains: "Are they challenging themselves enough?"
Peter