I am a composer, Bach specialist and harpsichord maker. In 1984 while driving a truck, a theme pair came to me. Tovey's 'Companion to the Art of the Fugue' remarks on the difficulty of finding a countersubject for an ascending chromatic theme. I knew immediately what I had. It felt like Bach's ghost had told me: "I overlooked this theme pair during my own time, see what you can do with them".
I wrote out the exposition that night between chess games. Completing the fugue took another five years. You may hear it here: http://cdbaby.com/cd/stevejared2