It seems to me that a multipurpose daughter board that contains a good mix of capabilities and was suitable for a wide range of uses is really important.
Clearly, servo outputs and standard sensors such as 3 axis Accels and Gyros, magnetometer, barometer and a GPS connection are necessities.
However, given the extreme capabilities of this board and it's propensity for dealing with image and spatial processing in a highly capable fashion perhaps some consideration could be given to a high performance camera / video / spatial interface: Maybe a Dual frame buffer with direct Multiport FPGA access, possibly configurable as 2D color or 3D point cloud and dynamically binnable.
Seems like it would be best to provide a flexible format so any of a wide variety of input devices (PrimeSense / Leap, TOF Cameras, Laser Scanners, Video Cameras, Stereo laser rangefinders, etc could be used to fill the frame buffer(s).
Just a thought, it does seem like the main thrust of this board is to let your system be able to see and to figure out what to do with what it sees.