Just realized this but the June survey specifically asked for what you would like in "updates", meaning its likely not DLC. Of those, there were 7 pieces of story content: Ardyn's past, Luna's activities, Cor's activities, Ifrits betrayal, The world of ruin, Noctis's disappearance (which sounds heavily like it could be the omen dream) and the line of lucis (which sounds like some Dawn 2.0 stuff).
The trailer we saw could very well be part of Luna's Activities and Ifrit's Betrayal. We could very well see more Cor cutscenes after chapter 9, Ardyn scenes of his tragic downfall throughout the game, World of Ruin content in Chapter 14, Noctis's dream/disappearance inside the crystal after chapter 13, and potentially flashbacks on the line of Lucis as part of this package.
Thats not to say that Luna DLC or Ardyn DLC/Expansion/etc. won't still happen, I just find it interesting that they called it "Story Content" back in June and it seems like that is what we may be getting with this. That still leaves some gameplay updates, like new spells and armiger abilities, so I'm curious as to what they could be doing there.
The biggest question I have is how "Big" this whole thing actually is.