You should consider a size as well. Something that can fit a basic project box would make it easier to get common parts.
Consider the buttons also. Without a good board design and enclosure, the geometry of the buttons and placement will be have to be custom build, deviating from the "kit" mentality.
What about the wiring? How should someone connect to the automobile? You wouldn't want it hardwired in. That dictates some common form of connector, what kind, how much, how easy to get, etc... This will allow pulling it for upgrades, or use in a different vehicle.
How to program the Guino, with a pass through connector to the enclosure? If not, maybe some way to mount the Guino so that it easily snaps out of the enclosure for reprogramming.
I don't think it will ever be as small as a SGII, but we should at least try. Looking at my pictures, outside of the Arduino and LCD, my components are quite small, a PCB would help here. My homemade connectors for easy dis/connect are large because I didn't have something more appropriate.
A good PCB would go along way to shrinking my whole unit.
With the exception of the LCD, buttons and inputs, would one of the generic project/proto shields work? Perhaps a custom shield? You would still need some custom connector to the LCD and external buttons. But, since those will mount to the enclosure, maybe it's better that way.
This is definitely a good idea, and has sorta been my perspective from the beginning. But, maybe a DIY-scrounge-for-yourself-on-the-cheap or a more easily utilized kitted version. Worth investigating once it is stable and usable, which seems to be now,
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