I would want to have it cost much less than an Arduino in a "DIP package." The basic logic can be implemented in a $10 FPGA, add on cost of boards and connector

I would want to have it cost much less than an Arduino in a "DIP package." The basic logic can be implemented in a $10 FPGA, add on cost of boards and connectors and a voltage regulator and it should still be less than $20. The advantage here would be that the user would be completely free to create her own "computer mainboard" with whatever peripherals she chooses. I might also consider doing an all-in-one with common peripherals, and if I did, a CDC/UART type thing would be high on the list. HDMI is easy to implement in recent FPGAs (easier than composite/S-Video/RGB out) which is why I'd want to display video using HDMI.