It's not an uninteresting idea, but it may be a hard requirement to pin down enough to make something marketable at prices comparable to stand alone FPGA and microcontroller boards.
Occasionally you will see a processor board accessorized with an FPGA, but in the only example of this I ever used, versions with the FPGA actually populated had a huge leadtime, so we ended up putting a high end CPLD on a carrier board that was needed anyway.
Often once a project goes the FPGA route, the specific FPGA becomes fairly dominant in the selection critera, and the microcontroller pushed to the background, as the FPGA is often there to provide peripherals not available on-chip, and this makes details of the chosen micro less important. This may lead to:
- A processor core being integrated in the FPGA itself
- the FPGA loosely coupled to a separate processor board, often via SPI (since that's easy to implement in an FPGA, though I've also done asynch serial by implementing a UART in a case where it made more sense)
- No microcontroller, with the FPGA instead connected to a PC, via SPI (bitbang on the parallel port, etc), or a USB interface chip, or USB micro. Increasingly today, you could use a highly capable arm embedded controller (rasberry, beagle, etc) in place of the PC, but like a PC still int he category of hardware you don't want to respin unless you have to or have huge volumes.