there really are very few form-factors that could be considered. here's where i've exhaustively evaluated all the ones that i could find: http://elinux.org/Embedded_Open_Modular_Architecture

the A20 computer card has Micro-HDMI and Micro-USB-OTG and Micro-SD. it's effectively the same as one of those USB dongle computers in that regard, so we're providing the option for people to pledge for a "cable kit". take care if you're going to source your own! make damn sure it's the right type, you want one that's "OTG Host Power" - many of the ones that you can find online DO NOT do the bidirectional power provision that you need.