Hello! I posted this earlier to the Testing Unity Session thread but this seems to be a more appropriate place, so I will post it again.
This is a mock-up I made for how the Yaru theme can be implemented. I based it on what is already working in the basic Unity support of the official Yaru theme, but filled in the missing elements, like the title-bar; close, minimise, maximise buttons; and different drop-down menus.
The icons on the top bar are mostly from the unrealised Suru. Some, like the battery, network and keyboard layout icons, I think may be mixed in from the old Ubuntu Mono Dark set, but modified to be of only plain colour, without any shadows.
Folders and symbolic icons here are, of course, the great Yaru icons, but for the rest I propose to stick to the regular gnome icons, without replacing any 3rd application icons either. This is similar to how the icon set was in Unity, and I think it is better overall, than the current Yaru set, as the Gnome icons are quite nice and mix well with other shapes and colours. Additionally some Gnome icons may be combined with the Humanity or Yaru elements, like the sound file and the html link icons here (as they are somewhat lacking as well).
The Launcher here no longer has those squircles for a nicer flat look.
Nemo is pictured here as the main FM, because I think it suits Unity better, having proper menus and other expected functionality. It feels more like an improvement upon what the Nautilus was in 16.04. Besides, it is already being used to draw the desktop icons.
I also added a bit of restyling for the very nice program called Gnome Sushi here, just in case.