One of the things that has always confused me is that Gnome Shell is
really extendible, from a programmer point of view. Even if you don't like Gnome Shell, you can actually start with mutter and build a window manager really easily (or, at least this was the case about 5 years ago when I last looked into this). Building Unity on top of Gnome Sell should be
very straight forward. I'm still surprised nobody has tried to do it.
Again, I haven't looked at it lately, but the last time I did, mutter was just fantastic. I was tempted to write my own WM on top of it, but since I have grown disenfranchised with the rest of the Gnome ecosystem (and it's hard to pull that out of mutter), I gave up. This shouldn't be a problem with Unity, though, as it is already entrenched in Gnome.