Andys newest controller (U-Hid) supports almost any configuration you can imagine... from multiple trackballs, multiple spinners, multiple pedals / multiple pots, rotary joystick support, and even can output voltage/leds.
At $80, its not cheap... but for what it does, its quite awesome... and I cant imagine much improving upon that level of customizable functionality.
My post wasnt meant to be an attack... merely a question of what direction you were intending.. and how you were going to compete in a crowded interface market.