I think that course designers gain experience through watching others courses that are built and learning about what parts of the courses are challenging for the riders. They also watch their courses being executed and learn from that. I think that is how they find new ideas. For example, in a Grand Prix they don't want the courses to be easy. They can challenge the riders by setting the lines inbetween strides so it walks at 6 1/2 not a 6 or 7. They can also have a plaque after the water jump so the horse has to get back on their hind end after reaching out to clear the water and clear the plaque. Or a tight double vertical in and out. Or a steady line/ long line. There are many ideas out there that they can use and mix up courses they design with. Hope this helps!
For me one of the best track chief is the Italian Uliano Vezzani !! I know he takes care of the Longines Masters and these courses are great. I know that the hardest part for ski runners is to make routes that are always more spectacular. It is also necessary to entertain the public but especially the riders and the horses who are almost every day on competitions !! A difficulty for them is also to create courses that do not repeat themselves ... For Uliano Vezzani, a course is like a stage where horses can express themselves 100%. The secret to being able to create tracks is to have a lot of imagination and fantasy.