There are a variety of development boards (Livid Brain, uCApps MIDIbox) designed to allow semi-technical users to build their own MIDI controllers. It's also reasonably straightforward to configure an Arduino Micro or a Teensy to work as a USB HID interface. If you can use a soldering iron and a drill, you can build a custom controller.
Absolutely. I've been known to break out the breadboard to that end. But soldering irons and arduinos are a level of too much... erm... wires, for what I'm thinking. Not to mention that making it control what you want takes time and debugging and perhaps a sympathetic operating system or app.
How about forgetting the electronics, and seeing controllers as just another UI problem. Software, implemented in plastic.
What would you want to build if it were that easy? Custom controllers for games? Custom games? MIDI Instruments? Dashboards with physical elements? Home / media center controls? POS interfaces? Hardware usability testing prototypes? Kitchen gadgets?
Maybe I'd be the only market, but I think it would be a big blurring of the line between hacker and maker culture.
