While we’re talking about a UI redesign, I’d also like to see:
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More visual separation between the areas of the interface, especially between the dockers. This is my big complaint on visual UI, and part of the reason I think Blender’s interface looks sleek. Makes it easier to scan for dockers.
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Theme customization would be nice. None of the existing themes do exactly what I want. Krita Darker is the closest, but the scrollbars have no contrast, which is my biggest peeve on the themes (the Krita blender theme, interestingly, is much brighter than Blender and unfortunately renders black text on gray, which is sometimes squint-worthy).
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Icon coloration for easier visual navigation (the Blender 2.79 way) or make it possible to switch between icon sets (Blender going forward). Colorized icons can be done programmatically even across themes if the colors are determined in a perception-adjusted colorspace so that visual contrast doesn’t change as you cycle through hues.
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Removing the outline around the undock and close docker buttons would be easier to process visually, I think. I couldn’t tell what these were when I opened Krita the first few times:

I’m seeing other possible UI issues as I go through the UI to write this, which I’ll try to remember to submit as a separate question tomorrow. Meanwhile, I’d love to know if there’s a place where someone is collecting these thoughts on UI so people can talk about it.