AppOpen-ads already exist in the admob plugin, they are just not exposed in the C3-plugin. But you can call the function by a js oneliner directly in the event sheet.
(It does not really work well in a cordova/www app though, AppOpenAd should be visible while the app is loading, at splash screen basically. but if you call an AppOpenAd from the event sheet it will be shown after the project is loaded and already running, so this is not really how it is supposed to work.)