what would your first hardware ad-on be?
An Arduino. 
Because I don't want by 1GHz-class CPU having to bit-bang I2C (400kHz) out some GPIO pins.
First prize to whoever reconfigures a Rasberry-pi or similar board to be an "ethernet shield" for an Arduino!
(that may be the best way to get advanced networking features like IPV6, IPSEC, SSL, etc. And it's cheaper than the existing ethernet shield...)