An adapter board that will convert a standard 5V Pro Mini (or 3.3V) into the standard Arduino Uno shield layout.
The shield should include a built in FTDI adapter and voltage regulator so that it functions just like an Arduino Uno. The Pro mini could clamp down using special connectors or use pogo pins to make the electrical connections. There should be a switch between powering the board with 3.3V or 5V (applies to the FTDI also). The input and output pins should be left alone and not include any logic level shifting.