I hope Wayland takes this opportunity to solve multiseat once and for all. X has struggled for years trying to support every theoretical combination under the sun and it's still a nightmare to configure.
It might be best to focus on one or two common use cases in the modern age.
Lounge Room Use Case : Plugging in a HDMI TV via a second GPU port, and having that display automatically stay logged into a media center or guest account with a wireless kb/mouse controlling it. All the while the primary display can have a rotation of accounts logging on and off again.
I'd like to see Wayland solve that for the majority of users, and then drive from that towards additional capabilities.