Amusingly (?) Avid's Carbon web page is currently missing in action, throwing a '404. Oh what does that mean? Somebody screwing up? Somebody trying to update it?
https://www.avid.com/products/pro-tools-carbon
But as for more expansion, OK you can get ADAT, but why would Avid produce their own ADAT preamp? Piling into an already very crowded market? Even if they made it with ADAT and AVB. ... but could it have DSP on board and span Hybrid DSP across boxes? If so then the economic development thing to to do for Avid would be to just support multiple Carbon connected at the same time?
(No not that I am personally going to buy a Carbon, I have a RME Fireface UFX+ on order... if RME ever gets their delivery act together).