I have posted and discussed over on the forums a set of designs for the window decoration buttons that I think are more intelligible than the default 'Oxygen' style used in Breeze.
The existing Breeze window controls (and Oxygen since KDE4) are not visually descriptive of their function; the 'diamond' especially being confusing and non-descriptive. There is also not a clear distinction between the ambiguous down and up icons for minimize and maximize respectively, and the diamond (I believe the 'diamond' is actually supposed to be like the two arrows used in Windows 3.1, but that is not clear). Several users who I have introduced KDE to, have been put off by these unintuitive, unfamiliar and unorthodox icons; I believe changing them would help KDE's adoption rate. Even after some time of use, occasionally I even click the wrong icon; the problem is made worse with GTK apps with drop-down menus in the title bars that look identical to the Breeze/Oxygen minimize icon. There are already many uses of arrows within the desktop environment, so also having them in the titlebar of every window where they point to nowhere does not help clarity.
I would therefore propose to replace the Breeze/Oxygen titlebar icon style with a style reminiscent of that used in KDE1. I have put together such a style which I have named 'Classik' for now. A preview of 'Classik' (made by modifying the C++ QPainter calls for Breeze kdecoration and kstyle) is available in the screenshot below:
The 'Classik' minimize icon visually describes a compact icon, like the appearance of the minimized window in the task manager/panel. The maximize icon shows a large square representing a maximized window, the restore icon shows a visual representation of floating windows. I have also updated the Keep Above and Keep Below icons to be consistent with similar Breeze application icons.
I have also made an Aurorae theme of 'Classik' which you can preview. However, Aurorae is more limited in that it does not accept the system colours for titlebar, does not theme window controls within applications (like kstyle), does not update GTK themes and does not update the system icons. I therefore think a fuller implementation of 'Classik' than this would be a better Plasma default.