Building on that:
The notification center should have other functions such as video thumbnail and the possibility of answering messages and emails.
Messages and emails are still just notifications, in accordance with the Desktop Notifications Specification. I have not yet seen any specific support in the spec for the kinds of notifications you're thinking of (with the ability to reply). You're able to specify a limited set of actions, like clicking a "Reply" button which would open up the application to that view, but other than that it isn't supported in the spec.
Video thumbnail is something I don't see any value-add in. You're not going to be watching a video in Raven. There are various Raven widgets which are planned such as Weather, Stocks, and RSS ("News"), but to do that there needs to be a Raven API, which is targeted for Budgie 11.