Hi Matt,
thank you for inviting the community for a discussion!
I understand your excitement and using a FPGA to get around the limits of the existing IO interface is a really sophisticated way.
I've another fear (i'm german, we have always fears if it comes to technology): Software and documentation support. Get me straight: As you see here in the forum, a lot of Galileo users have an Arduino background and struggle "already" to transfer there knowledge (and sketches) from Arduino to Galileo. Some popular Arduino sketches, libraries and solutions might be only usable with your proposed shield - but telling these users "just program the FPGA as needed" is not an option. You should not only plan the hardware but also spend a lot of time to provide the neccessary knowledge and software tools for non-professional users.