Such a thing would probably see you have to learn a fair bit of electronics, a fair bit of programming and a fair bit about midi if you were going to go manual.
Instead I would consider picking up a teensy++ and fiddling with something like
http://www.pjrc.com/teensy/td_midi.html
What you want your midi controller to ultimately be remains up to you but you can pick up things like
https://www.sparkfun.com/products/7835 and variable resistors are not exactly hard to come by either.