Pattern Drawing and Composition using basic Art Rules for layout, perspective, focal point placements, how the human eye perceives a design when first viewed, color choices and compatibility. These are very lacking in any stained glass class. Everyone assumes you either have the knowledge or that your going to work out of a pattern book the rest of your life. I belong to several Facebook Glass Groups, and see some of the self designed patters there, and in most, there is no discipline in any Art skills or they are just non existent. Folks can not seem to get it in their heads that there is no difference in creating a Painting, or a Stained Glass piece, other than the Medium used. One you paint in with oils or acrylics, and the other you paint in with glass. I have always told those interested in doing glass, to go out and buy a good basic Art Book, and study it well before trying to do your own creation project.
Anyone can be taught how to use a tool, and how to use the material, and can create a "Cartoon" type panel, but not everyone knows how to create a work of art. To do so, first requires the basic knowledge an Artist needs to have. This is not being taught as far as I have seen. Might be a good place to start, using a "Scenic" panel to start.