You already can configure transparent menus in the settings of the Breeze application-style, so why don’t add a similar setting that makes the windows transparent too?
You already can achieve transparent windows, but you need to do that with Kvantum, since transparent color-shemes result on not blurred transparent windows with flickering background. Furthermore I noticed that there are much more bugs, if you are not using the Breeze-style.
Another idea would be making transparent windows and menus the standard, since they look much prettier (in my eyes)
Transparency is “supported” by breeze but as you can see bellow… it’s not quite well supported as it’s only work with kirigami apps (and look ugly). It can be done by tweaking color schemes in ~/.locale/share/color-schemes
I’m assuming you’re on a 6-ish? If so, I’d like to see what it looks like, (with an edited color scheme) for transparency to work with standard breeze application style on 6-ish.
It looks like that which I have posted before (Breeze + Alpha color-scheme), but you need third-party extensions to turn blur on an the bugs shown off.
So I think it should be officially supported by Breeze.