Can’t wait to test it 
Quick notes to the mockups:
Shadows and inner shadows are too strong and wide, also I’m very unsure about the logo. Maybe white only and not sure if the thought behind it is right. I did try the logo on the top panel instead of the arrow but also unsure because how the button works…
The font seems good, also the Popover looks good, I just don’t like the borders too much on the buttons and in the menu layers under the text (Shell focus states gif).
Maximized window was also a big point for me because it should (for me) not melt together with the top panel. That was also the reason why I kept the shadow and in the new mockups, I tried a lighter grey for the windows to have a better visual separation.
I also was using the blue color for testing from the Suru theme, but I changed it back to orange because it just doesn’t fit somehow…
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New thoughts, new mockups:
Because the last version was way too dark I took a step back and thought “How can we modernize Ubuntu without losing the typical feel and look while having a system that works for millions of people?” That is a big responsibility. And it’s a little more conservative, unfortunately. So dark theme comes later.
The main thought was, that the dark/gray is good for a background/frame, but at the same time, the icons must have a surrounding to hold a certain contrast that does not make one’s eyes tired over time. So instead of dark, I was working now with light layers.
Yes, it does look like a modernized version of Ubuntu. And more conservative. But the eyes can find everything fast.
Maximized windows separate from the top, the colors are maybe not perfect and the little Ubuntu logo as a test in the right corner.
And at last, I was testing the Suru elements:
They look neat and nice and they fit the design, only the blue that they used just does not work for me and don’t fit to the rest of the system (and is more like Gnome^^). I want them to work on dark and light background, so they have sometimes a little shadow or a certain grey - but this is not finished yet.