RS-485 + iCE40 for better CAN-FD
I am cooking up an RS485 based protocol for building automation; single master, 250kbps. Nothing else quite fits my needs. It lets me create the tiny, low cost sensor/actuators I need. Reinventing the wheel can be OK IMO.

The CAN bus physical layer supports collisions which is essential to its support for multiple masters. The RS485 physical layer was not meant to handle collisions but more recent drivers and receivers behave gracefully in fault conditions, so you could make it work. But have you considered using a CAN-FD transceiver for your custom protocol?

You might be able to create something similar to your FPGA concept by dedicating a reasonably fast ARM Cortex MCU. Might be easier to develop, could well be cheaper in production. The result may be easier for others to work on.

Finally, you mention 48Mbps. I was under the impression that both CAN-FD and RS485 top out at about 10Mbps?