I know that currently Dolphin file manager is one of the best in the world Gnu / linux and from the point of view of design is not even that bad, but after seeing this Mockup of Sagatori for application for picture,(I Love this mockup)
http://i.imgur.com/W9BycOB.png
I thought it would be nice to have a polishing Dolphin with a style similar to this, by the way I think is not even very difficult compared to other programs, so even if is it not a priority, I'd like to see some ideas about it. Sorry if the request is for something unplanned, only the request of a user

I kind of agree with you. I'm no designer, but i somehow i get the feeling (oh yeah, vague, i know) that things arent aligned properly. Mainly i think between the places sidebar and the main area where your directory structure is. Like here:
http://i.imgur.com/VWMMC0O.png

As i said, i am no designer so i dont know if those are supposed to be aligned. But it feels a bit.. crooked? Hope i get my point across here.

There also seems to be an issue with the context menu you get when right clicking the toolbar in dolphin. The selection color merges with the blue background of "toolbar settings" headline. (Cant seem to get a screenshot of this, sorry).

All minor issues of course. But i think lazyit has a good point in that it might need a bit of sprucing up

Hi KDE-Forum-members,
I did a little mockup for Dolphin. It was done in a few seconds, so please apologize that it's not perfect and features, like splitting the view or changing the view, are not considered by this mockup: http://img5.picload.org/image/ciacldp/dolphin.png.

A Dolphin designed like this would make use of the new ability of the Breeze window decorations, to do not draw separator in active windows if titlebar color match window background color, how it's described in the changelog (this is a feature, which the Muon software manager also could make use of in future releases). The client (that means, the widget theme) should draw the separator under the toolbar itself.

I didn't see a file manager mockup for Plasma 5 before in this forum, so I made one myself... I'm not a designer, and I don't have experience with UX-design, so please don't worry about the lack of perfection in my mockup, it's just an idea.