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.
One potential problem is if Gnome make breaking changes, it's hard for Canonical (or anyone) to use it as a base for their own UI. I've not experienced this first-hand, since I haven't used Gnome or written GTK+ apps since the 2.x days, but judging by what I've seen online they like to break APIs at each release, drop working features if/when their opinions change, and be semi-hostile to any use-case other than their own default settings ("brand"), e.g.
This is actually the reason I gave up on my idea of working with Gnome Shell. But as far as I can tell, most of the badness happens before you get to Shell. Basically, Canonical has had to deal with it in Unity anyway.
> Building Unity on top of Gnome Sell should be very straight forward. I'm still surprised nobody has tried to do it.
I think this is what ElementaryOS[1] did. It was funny/interesting to me that the "flashiest" features demoed on the website were pretty much just stock Gnome 3 features.
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.