There are a lot of things that the ATmega CPU on an arduino could be programmed to do, relatively easily, that are a bit ... exclusive ... of having the arduino do other (normal) things. They might have tight timing requirements, use up large amounts of the available CPU time, use timers in ways that conflict with analogWrite(), and so on. Multiple servo motors, multiple serial ports, and large display (video or LED) refresh are typical examples. Yet the Arduino environment is known and understood, and it would seem a shame to require a special purpose shield to do the same thing that an arduino could do.
So... What do you think about putting the basic Arduino core (16MHz Atmega168/etc) ON A SHIELD. Essentially a "slave" arduino, or an arduino "co-processor" based on Arduino...
It would communicate with the main arduino by serial or SPI. It could be programmed by running a special "relay" sketch on the main arduino, or by ICSP, or by unplugging the 168 and plugging it into a real arduino. It would tap power from the main board, as per most shields, and replace the power and usb areas of the normal arduino with a prototyping area for additional circuitry. The PCB could double as a "reduced" arduino "runtime board." Ardunio shield connectors would stack, and new connectors would show up "elsewhere." Even mere pin expansion would be reasonable; a shield board with an ATmega8 wouldn't be THAT much more expensive to build than a shield board with a couple if 74hc595s, and it would be a LOT more flexible.
What do you think ?