Daniel Jalkut recently wrote this piece about the Touch Bar coming to an Apple external keyboard. That would allow for the Touch Bar to be used with desktop Macs and MacBooks used in clamshell mode. Daniel thinks they’ll eventually get there.
I agree. I'm certain it would need more charging (and perhaps a bigger battery) than the current external keyboard and I’m guessing it will be pretty pricey but I fully expect a Touch Bar enabled external keyboard to eventually show up from Apple.