I agree, adding a FPGA is not an easy work, is necessary add an external connector with many pins, I like your idea, similar to David Reyes sugestion. If we using Nano as brain, and enable some ports for external use, like I2C, SPI, USB host, we can design a lot of interesting applications.
I think that a good Idea woul be make accessible the data, address and control processor bus, so, the people can build custom boards with any FPGA.