I think some of the first things to change should be the icons:
"start-here" (the Ubuntu logo), and the window decoration icons (close, max, min). They should use system wide icons, rather than Unity-specific icons. The Ubuntu logo can just reference "start-here" icon, or similar, and the window decoration should extract icons from the Window Manager theme.
The next things to implement should be the GTK+ theme's settings for the panel: transparency, BG colour, BG image, font settings, etc
This will also make Unity more portable to other distros, and more customizable like gnome-shell.
Regarding the Launcher, you may be able to use some features of GTK designed for gnome-shell, or GTK may accept some polarimeters designed for Unity specifically - so theme designers could specify different properties to allow the same theme to be used both for Unity, GNOME-shell and GNOME2.
I think some of the first things to change should be the icons:
"start-here" (the Ubuntu logo), and the window decoration icons (close, max, min). They should use system wide icons, rather than Unity-specific icons. The Ubuntu logo can just reference "start-here" icon, or similar, and the window decoration should extract icons from the Window Manager theme.
The next things to implement should be the GTK+ theme's settings for the panel: transparency, BG colour, BG image, font settings, etc
This will also make Unity more portable to other distros, and more customizable like gnome-shell.
Regarding the Launcher, you may be able to use some features of GTK designed for gnome-shell, or GTK may accept some polarimeters designed for Unity specifically - so theme designers could specify different properties to allow the same theme to be used both for Unity, GNOME-shell and GNOME2.