Here are some ideas for hardware extensions of the bitbox :

OUTPUT

• LED BAR interface : big flash when you loose or just for an ambilight (I like this one very much), maybe some ws2812 LEDs (or LED matrix)
• mini external second screen for an external screen like the nokia 3310 screen (cheap !)
• Shock collar interface (just kidding) - http://www.tomshardware.com/news/Sega-Genesis-Shock-Collars-Video-Furrtek,16633.html
• relay+FANs
• motor control (robot ?) other toys ...

INPUT

• MIDI in (with optocouplers)
• Interfacing with nice components (such as sensors, e.g. accelerometers)
• Simple camera
• Microphone
• Simple input stage with contact elements
• multiple (??)

I/O

• PS2 keyboard
• second CPU ?
• Arduino ?

Other ideas there : http://playground.arduino.cc/Main/InterfacingWithHardware

The DVK501 board could be interfaced: http://www.waveshare.com/dvk501.htm