>> A suggestion: start with providing just digital interfaces: ADAT, and
How about a standard adc stage with FPGA processing/routing and I2S bus.
For ADAT there are already the OPTOGEN and OPTOREC modules (dont have the links right now). Then ADAT and spdiff/aes-ebu as standard interfaces.
An optional XMOS low cost board for avb, which is capable of I2S.
And a FPGA extension board for MADI and ieee1394 also connected via I2S.
I kinda like the idea... could be highly flexible...