Have you already thought about something like a block based editor?
Every block can be a certain kind. This could also facilitate remixing, you just pull other blocks in and out.
For the preview I would expect it to always be the full document as it is later rendered on the client  

 Yeah we'll have to get creative for displaying and allowing the user to manipulate the blocks, but I think that's the direction we're going.

The editor will be the first really sophisticated part of the UI.