I agree about System X being too plain. It takes a trainload of furniture, people and plants to make it look habitable.
It would be great if the walls came in different colors and they gave us more options for window inserts as to shapes and color. And some trim to cover the connector piece holes would be lovely. I am thinking of something based on Frank Lloyd Wright or Motawi tile designs that would dress up that trim or floor edge piece at the top of every wall.
And it is so difficult to find 4x6 baseplates to finish any System X building of a decent size. Our dining table is covered by an unfinished set of boutique shops because I can't find large floor baseplates to finish the upstairs. Ugh.
Cut I still like System X and especially the lovely wedding church. Cheers, Pgal