A great addition to the DC motor shield would be a tiny duino or lily shield that encodes absolute angular position using the hall-effect based AS5048B.  It determines the angle to 14 bits of resolution or approximately .022 degrees. The chip is $13 single unit quantity, but the eval board is $220.  Seems like an opprtunity.  Stack one of those on a tinyduino board a with a motor control shield and you'd have a great stack.