Hey Phillip...what are the chances you'd consider producing a pattern design course? I suspect I'm not the only one who would be thrilled to see your tips, tricks and techniques to go from a concept in your mind to a functional pattern.
The De Havilland bag would be cool but really any non-flat design (that requires working things out in 3 dimensions) would be amazing.
Fashionary published a book of bag designs, but I don't know how to transfer the designs to a pattern accurately. @Fadi mentioned using CAD. Another leathercrafter I know uses Illustrator. Which is the best design program to use in our context as craftspeople first, designers second?