I just stumbled across the design work being done on the upcoming KDE 5 release, and I have to say that it's looking pretty good! I knew they'd go with flat design on this one, and I'm so glad they did! This is just another take on what they're doing, so far.
This is a WIP. If there is interest, I'll release it when it's done.
You can check out the work being done on KDE 5, or even participate (yes, you read that right!), here: http://wheeldesign.blogspot.se/