There are some shipping products that use M.2 with with PCI-e x4 but they live on their own PCI-e cards since there were no shipping motherboards with an x4 M.2 slot at the time. They also tend to be sold to OEMs only for the moment. But once the newer chipsets are well established that should change.
The streaming engine really peaks my curiosity. I personally don't stream or capture gameplay (I am not that good), but with the rise of twitch this might be a great investment for those guys. Don't have to bother with the capture card anymore, there is one right on the motherboard.