I am keen to get support for multicolor printing out there, so I have been working on an extruder expansion board.
The initial concept is for 4 extruders (stepper drivers, heater outputs and thermistor inputs).
The interface is via I2C for ADC and serial IO (SPI compatible) to set outputs. It could be scaled up to 6 or even 8 extruders quite easily, though I would consider using a microcontroller for 8.
It is compatible with 3.3V or 5V logic, so could be used with any system with some spare IO.
Let me know what you think!