Running a Desktop Environment in a system container looks promising to me. @tarruda wrote the tutorial and I got it working in Incus.
The problem with this approach is that it relies on X11 and Spice. The general trend, at least with Gnome, is to move away and deprecate the X11 components in favour of Wayland, and in that aspect, Wayland has not been a major priority for remoting software due to lack of protocol components. Gnome and KDE do ship a built-in RDP server however.
While Gnome has backtracked, the first proposals for Gnome 49 was the complete removal of X11 components in their DE.
It also remains to be seen how actively Spice will be maintained since Redhat deprecated it since 8.3. The mailing list archives don’t paint a great picture nor does their Gitlab activity. Even Proxmox is looking to migrate away from Spice: > I wonder if tools like Proxmox could use this as a more efficient alternative ... | Hacker News
It’s also as you said that the components are there, but the glue does not. You can pass through the host’s Wayland and X11 sockets to the guest but configuration of such is left as an exercise to the user. My current attempts have largely resulted in working Wayland and X11 programs but not theming integration; personal future exploration is to look at nested KDE compositor inside a system container.
I can’t speak for Gnome but KDE also has a documentation bit that shows how to run a nested KDE Wayland session. But not much regarding headless Wayland hosts.
There are also misconceptions in your comment, Incus integrates Qemu into its stack much like how VMM (edit: VMM as in Virtual Machine Manager, the former Redhat project) does with Qemu. If you’re mainly interested in a GUI solution now, then you can take a look into Gnome Boxes (or the equivalent KDE project that eludes me at the moment.)