But what was interesting to me was the fact that you could add a peripheral that was not initially intended by the manufacturers, making a mainstream motherboard more versatile.
For you it may be a video transcoding unit, for someone else it may be an SPI or I2C device, PCIe, or extra ethernet, or high quality audio.
I'm not sure what peripherals were implemented by the community for the RP2040 either, maybe they would not make sense on a PC.
It’s not cheap, and you need to write device drivers etc - but that’s the case regardless.
Though there is a small difference in my opinion, where, from the point of view of the CPU, it should behave as a normal interface, thus the driver should already exist, and only require a change in the device tree (for linux).
It would still require quite a bit of work: - The PIO has to behave bug-for-bug compatible with an existing driver - The exposed pins need have the proper voltage levels & electrical protection
.. but that's just fantasy for now.
there are a few FPGA dev kits which are set up this way. I have one which has a dual core Atom CPU and a large FPGA connected via PCIe, and the speed is fast.
Well, someone implemented bit-banging Ethernet TX: https://news.ycombinator.com/item?id=35810281