You get the odd one for disabled people, a few for developers that need sensing and the rapid fire/macro mod type things (see something like the evil controllers), and whatever keyboard and mouse mappers are doing this week. Something more custom like make these contoured section bigger, smaller, 10 degrees more inward turn, scale by ?%, relocate triggers/buttons... would be doable but no gamer would ever pay the money for it -- current controllers are either custom enough or locked down enough that you get to cannibalise one of those before doing anything, add in a few hours to do the CAD and play with a (good) 3d printer and it is not really viable, especially for something as personal as a controller (
http://theoatmeal.com/comics/design_hell is so very true).