I think certain of the remote protocol implementations have the ability to forward a single window, for example seamless RDP.
It would be good if someone knowledgeable about the state-of-the-art available in competing remote protocols could shake their cystalball and peer in the not-to-distance-future of 2 to 4 years from now (the timescale for Wayland dominance) and try to paint a picture of what should be possible. SPICE and RDP being the obvious candidates that leap to mind for me. I think some of us curmudgeony people instinctively think VNC is meant to fill this role because its been the workhorse for many of us for a long time (too long maybe if the old dogs and new tricks adage is true.) But maybe that's the wrong technology to slot in as a puzzle piece here.