Greg - Not to drive (poor choice of terms --hehe) the design - and backing up to the number of pins discussion -- you could add a second parallel port and increase the the numbers of pins - but it probably doubles the costs of that section of the board! but greatly enhance the modularity aspects.
Regarding the type of motor - bipolar, unipolar, servo - you'll "know" what meets your needs and sharing the results with all will be most kind - and keeping within the OpenSource -- thanks for that!
AND, certainly at some point you just gotta say, "Pencils down - time to turn in your papers!"
Great effort in creating the spec and good luck! I'll most likely be saying later, "Wish I had one of those"
Cheers - Jim
Experience is the BEST Teacher. Is that why it usually arrives in a shower of sparks, flash of light, loud bang, a cloud of smoke, AND -- a BILL to pay? You usually get it -- just after you need it.