Rich templated notifications are now available through the new Notification Center
in the last Windows chromium build available at download-chromium.appspot.com/dl/Win
To see them in action, you need to toggle "Enable Rich Notifications" flag in chrome://flags
Screenshot below shows you how to create a notification within a chrome extension but this also works with regular HTML5 notifications.
As you probably guessed, this Notification Center will be the perfect hub for Google Now cards.