...a slightly larger Duemilanove-format PCB with some extra circuits on it.
Is there a standard practice for designing compatibles that have some pins consumed? Heck, is there any standard practice for designing compatibles at all? By "compatible" I mean "works with most shields." Maybe I'm not using the right word though. . .
A bunch of people are building RTCs and EEPROMs into their boards, usually (always?) via i2c, so only two pins needed (A4, A5). Motor controllers seem to be pretty flexible in terms of pins, depending on whether PWM is needed.What sort of extra circuits are you thinking of? Might help to figure out how to handle pins.