I really like the look of Ubuntu 17.10 + GNOME stock icons, but there’s clearly a lot of momentum here to make Ubuntu look less like macOS (gradients and realistic icons) and more like Android (flat colors and uniform icon shapes). Please keep usability in mind if Ubuntu really makes the jump from one style to the other.
Example: Ambiance makes it really clear which window has the focus – its title bar has a bright gradient, whereas background windows use a flat dark gray. I find it much harder to tell which window is focused with Arc and other flat themes.
The Suru icon theme and others also remind me of minimalist movie posters: They’re really subtle and clever if you know what you’re looking at, but I don’t think they’re self-explanatory enough for newcomers. For example, the calendar icon is a roundrect with a number, whereas GNOME’s icon is a lot more detailed. There’s also inherently more “distinctness” that icons can have when their shapes are wildly different, which makes it easier to scan a list of applications.
I don’t have any skills that I can offer, but if there’s something like a Patreon for Linux usability tests, I’d love to chip in.