Xfwm4's tabwin/window cycling looks a bit old-school today, a refresh of its look and the possibility to theme it would be desirable.

If you wanna test this branch, pull and build it. You need libxfce4ui from git-master installed as well. As an addon, you can pull the Greybird-theme with some tweaks that make the tabwin look like on the screenshot above: https://github.com/shimmerproject/Greybird/tree/ochosi/tabwin

This is pretty obvious, I guess.

(Note: what's missing in this mockup is a numeric indicator on the top right of the window_button that shows how many windows of that app are open/grouped.)