I love the human theme! Its terrific, but anything can be improved. I think Ubuntu should keep the general them of Human, but just evolve it.

Specifically, the human theme could benefit from improved depth and increased vibrancy, especially on items that require focus (like selected items).

Some example improvements:
*Add more shadows, especially on components that are inside applications
*Have a higher-contrast pallet for selected items (ex: drop-down menus)
*Decrease the contrast of window outlines
*Replace the two-tone base gradient of title bars with a more complex highlight to saturated to unsaturated gradient
*Increase contrast of deselected window title bars
*In addition to brown and orange, add another color to the pallet which is used very sparsely as a highlight (ex: this website uses bright red -- however, I think a highly saturated blue-purple would contrast better)