As an example to what you can do when combining existing controllers I present the Def-Con 1 ("Definable Controller") which is a combination between a Belkin Nostromo Speedpad n52 gaming controller, Mixman Dm2 and M-Audio Trigger Finger hacked into a custom made and laser etched box.
Click for full image.
All 3 devices work as normal according to the computer and are connected from the 3 individual usb cables via a usb hub. They are each programmed in their own respective software (M-Audio Enigma, Nostromo utility and DM2Midi). Because all 3 devices are fully programmable I also use this controller for VJamm, Live and Traktor. The mapping gets tweaked as time goes on but the one shown above is the latest. This controller was created by myself and Komega, creators of the Livepad/Kromatron.
As you can see it's a bit thematic with the russian cold-war esque design. There's even an ignition key (the keyhole is next to the white button)!
If anyone wants any more info just let me know, I'd be happy to share mapping files or any info on how stuff was made.
Hope this gives you all some cool ideas for custom controllers!