I think what the original poster had in mind was to build a device using Arduino that from a PC will look like a USB device that has 15+ potentiometers + 2 joystick axes.
I am currently thinking of a similar device (although with not so many analog controls but a analog outputs and few buttons).
first (and probably most easy) thing to do is to write a Ardunio sketch that reads the analog and digital signals. Then send them to the Atmega16U2 which has been re-programed to appear as a USB device to the PC. Look at this site for inspiration.http://hunt.net.nz/users/darran/weblog/a3599/
Where I am stuck at the moment, is the lack of information on how to compile the modified firmware (using LUFA and avr-gcc) that can be flashed to the Atemga16U2. I have asked that question in another post, awaiting the response.