An example of a complex piece is the sleeve. Our arms do so much in a day - they are rarely still and the shoulder and elbow make it even more intricate when it comes to design. On paper, the pattern for a sleeve looks pretty straightforward but once it is cut in fabric and sewn together and inserted into the main body (e.g., of a jacket or a dress), it is a completely different matter.
I enjoy researching fashion and works by other designers. Here is a sampling of intriguing ideas I came across on Pinterest. So many great ideas and shapes and textures!