[footprint]

Hi Jens,
Yes i do like your joystick mod its very cool and im thinking of doing something similar but using the UNO and sending it all over WiFi, i've been looking from some good cheap joysticks (2nd hand) but with little luck so far.
The ITX board would provide 2 things; USB so you can use one (or 2) of the 24ch servo controllers and an Arduino for just the digital outputs, lights, sensors - tie these into a user interface and you have unlimited mixing & control options, eg pump, lights and indicators.
And second easy WiFi, the Xbee mods are pretty good and coming down in price but 802n cards with pigtails will a give better range and antenna options.
So while you could use a Arduino Mega, (14pwm i/o) and an xbee, i would be more inclined to over complicate it for the additional features. At 30" in size i would guess there should be space to hide it all the PICO-ITX is 7x10cm, however as stated power consumption would be considerably higher.
Edit: you could also use 2 xbee mods and a servo controller that uses 232 via one xbee but im kind of looking at it like using the ITX as a PLC/SCADA which also means using a laptop as the controller.