the Touch Bar is essentially running watchOS on the T1 chip, which macOS then communicates with through an interconnected USB bridge that "relays multitouch events back to macOS."
It's interesting that communication with the Touch Bar is via USB. It seems to me that that would allow for an external keyboard to have a Touch Bar. (A wired one, anyway. Maybe even wireless.)
I wonder if something like this is coming to iMacs in the future. Might also be nice for MacMini and MacPro.