It could behave like tabs. Just allow to set task manager plasmoid to shown only entries related to app with active window, so if we had one window of Firefox and two of LOWritter, setting this option gives us opportunity to select Firefox in another task manager to show single Firefox window in task manager with described settings, or user could select one window of LOWritter and our task manager will show both entries for LOWritter window.
This could be great, because it could allow us to have something like tabs in each application.