What if they could compliment the TouchBar by simply turning the now super large trackpad (hint?), and the Magic Trackpad into a TouchPad with Apple Pencil support. And whatever window in a supported app you had active, could mirror, not unlike AirPlay, the contents of that window directly on to the TouchPad and you'd be able to manipulate it? It wouldn't have to mirror the entire macOS screen, just the contents of a particular window.
It would definitely be more cost effective than turning an entire 27-inch screen into a giant sized touch screen, which would surely jack up the price, similar to what the Surface Studio currently retails for. I'm pretty sure a full screen touchscreen iMac would cost more than a similarly specced Surface Studio. This approach would also keep in line with Apple's philosophy of having dual OS platforms.