Last time I looked for similar ARM based boards (larger ones at least), they were significantly more expensive (several times the price) and far larger.
Well, the Gumstix has an MMC/SD slot and 64MB of RAM onboard already, so OS wise (even if you don't like the pre-embedded 4MB Linux kernel) we're doing good. The daugherboard should probably have something along these lines:
I see since the last time I hit up Gumstix they've added their own breakout boards
A basic board would be breakout to 0.1" headers and a separated unregulated power hookup (screw terminals?) and a solid mounting configuration, but made as small as reasonable.
So... call it a wish list:
-All Serial ports brought to .1" headers, possibly with a jumper to send to RS232 if desired.
-Bypassable LDO regulator to provide power from unregulated supplies.
-GPIO fed into a CPLD (user reprogrammable), since the processor only has 20. Set it up for basic parallel port switching to increase the I/O available, but it can be user reprogrammed (possibly socketted for upgradability too, PLCC?) to do all sorts of tricks (hardware PWM and R/C servo controllers, hardware SPI/I2C control and buffering, etc). Heck, I'd compare the price of a CPLD vs low end FPGA's (I'm starting to learn about both). Might be cheaper and far more satisfying to drop in a Spartan3 to really let people go wild (socketted for upgradability, or soldered to possibly quadruple the number of available I/O, I'd actually go later and pick a common package so a "range" can be made available). Also a great excuse to learn about programmable logic. Might be overboard, might not, but that's why I'd say set it up as 16 I/O lines and 4 port selection lines (or similar) so people don't have to play with all this if they don't want to. I'm Xilinx partial at the moment
But I've only begun to scratch the surface of this stuff (I also have a half dozen CPLD's in PLCC-84 form from Atmel sitting in my samples box right now).
-Premounted or just pads to add on your own A/D or D/A converters.
I know, especially with the FPGA setup, I'm looking at a lot of horsepower. I'd have to see a rough cost estimate and expected hardware list before I'd commit (heck, right now, I'm not ready to commit to the Gumstix as I'm not ready to work on a centeral controller/UI for my stuff, not by many months).
It's pretty common for high end processors to have lots of pins for power and memory, but depend on a databus to get data out, so everything has to go to the chipset to get more I/O. The FPGA/CPLD setup would let you greatly multiply the I/O capability, and even design your own peripherals, or steal them from opencores.org and modify them to work with the system
Heck, crazy enough designer and big enough chip, you could easily integrate a secondary processor into the FPGA to take care of the mundane stuff. For the slightly less crazy, rerouting some of the signals to allow secondary FPGA control from an attached PIC or AVR might work, too