Nor does it need a "minimize-taskbar and minimize-tray action" in the protocol, this is handled entirely by the compositor. In fact, this could be handled without problems without even needing a "minimze" event. As as compositor, simply don't display the window surface, and place an icon in either the tray or taskbar: it's completely up to you.
Personally, I don't see a need for anything but a "window size changed" event. Why does an app need to know it's minimized? I think it should just carry on as normal. This would make window previews "just work", without any hacks.
For example, imagine you minimize some apps, and then press Alt+Tab and you want to select from them. If you send anything to the app to inform it that it has been minimized, it might do something that will make the app preview look bad. The app window preview being just its surface buffer that is displayed at another size.
But then again, it's entirely up to the compositor whether it wants to send applications the "minimize" event. If it thinks it might make them look bad, just don't sent it. I guess it doesn't harm anything to put the event in there.