A pinned tab + setting ctrl-tab to visit tabs in most recent order is pretty much superior - this addon's pop-out widget just displays the hangouts page, but it reloads every time you open it, so a very nice delay there. It also asks for the "read all webpage data" permission for some reason.

Unfortunately that seems to be what all these hangouts plugins do...

Maybe add some way to quickly respond to messages when you get one, like the respond-in-the-notification thing that Android apps tend to have? This would be somewhat complicated I imagine, but it would make for a killer extension.