That depends which accessories you want it to be compatible. Most other http://en.wikipedia.org/wiki/List_of_si ... _computers
have their own requirements and needs.
Raspberry Pi shields that that take the whole GPIO pin connector could be re-connected to some other boards (if you know which pins they actually use) and the some code would have to be written to control them. On the other hand if they have 5V GPIO then they can use cheaper and more common Arduino-compatible 5V boards (not shields) made by the Chinese in great numbers
I use pyMCU a lot (microcontroller board), and sometimes also pcDuino which also tries to be Python friendly for GPIO. Beaglebone Black has a lot on this subject too, but I didn't used it yet.