Since having a lot of windows open can be daunting or confusing to navagate through if they are on one workspace. I think having an icon on the windows open would make it much clearer to tell windows apart, whether it's to move windows to another workspace or closing them.

Fortunately enough an extension already exists that does this, but I would still love to see this functionality built into gnome-shell. Here's the extension: https://github.com/sustmi/gnome-shell-extensions-sustmi/tree/master/windowoverlay-icons