If you're going to use several different audio interfaces simultaneously (for recording/playback), they need to be running from a common clock-source... or the tracks will drift-apart over time (due to slight differences in digital clocks).

It's almost easier to input audio from older synths into a mixer or interface and record the midi channels. There's something to be said for hardware midi routing too. You could record audio and have a midi track at the same time.
CbB has sysex so it can speak to the older hardware.
 
Probably not what you wanted to hear, but music production has been concentrated on routing ITB for some time now.
 
The industry is focusing on the user who uses one or two input devices. Most stand alone usb recording devices are designed primarily for dumping audio that was recorded on location.
 
I know this isn't what you want and unfortunately you are in the minority. I don't think a dedicated MOBO for audio would sell because it would cost more and the market for it would be very small.

Setting up manufacturing for a motherboard can't be cheap.
The reason parts are reasonably priced; tens of thousands of each are made/sold.
A niche piece of hardware would be cost prohibitive
 
In relation to the original needs/concerns (extremely flexible routing/etc), both AVB and Dante are capable of extreme routing flexibility (many inputs/outputs, multiple-rooms, etc).  That (or digital mixer) might be a more practical solution.