Problem/Motivation
#1510544: Allow to preview content in an actual live environment greatly improved Drupal's content preview ability.
However, as pointed out in #1510544-131: Allow to preview content in an actual live environment and other comments, it's still not a completely accurate preview. Because the previewed content isn't actually saved anywhere, when you go to the new "full page" preview in Drupal 8, you won't see new menu items that were created for the content, it won't appear in views that it's supposed to appear in after it's saved, etc.
Proposed resolution
Change the "preview" functionality so that it actually works on saved drafts only (whether those are intentionally saved by the content creator via a "Save draft and preview" button or something more complicated involving autosave is TBD).
Use the forward-draft revision system so that a draft can still be saved in this instance when someone is editing already published content.
Modify Views and other systems to be able to treat these saved drafts as the actual published content, only in the case of a content creator who is viewing the site in "preview" mode.
Remaining tasks
All of them :)
User interface changes
No gigantic ones, but a few small ones likely as mentioned above.
API changes
Probably....