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.

That would still require a huge connector with a lot of pins. I can't think of an application where USB or SPI (I've driven it sometimes at 25MHz) would not suffice.

(Well, actually I can, but if one wants to build something with such a high bandwidth requirement to the CPU, you're better off using an embedded computer instead of a NN).

Un saludo.