On that note though, if they ever do anything with Gralea, I always said just covering how the 3 bros got out of Gralea after chapter 13 while being haunted by Ardyn would be the best route. It wouldnt alter or change anything about the main story. To find an airship or something. Smaller than the new insomnia, could even be linear for all I care.
The only other things I'd actually want in Niflheim are just more overall stops everywhere:
1. Altissia (believe it or not, there are signs that the cut trainstation is actually from here and the train sequence starts here; the railway connects to Niflheim via an underground tunnel, that's my guess anyway)
2. Succarpe (at least one relaxing town/area)
3. Cartanica (Let us at least walk to the quarry, the town is now known to be abandoned)
4. Eusciello (Abandoned Factory town with the whole desert/snow phenomenon, teases the eventual Shiva sequence)
5. Piztala (we stop here when Nilfhiem ambushes the train, but let us destroy the actual base)
6. Pagla (there's an actual train station here that the game skips, boot up Chapter 12 and you can see it almost immediately when it starts, it's a Cliffside/beach area)
7. Tenebrae (the only part where I hope they rewrite the story and let Noct get there before the Invasion; a decent sized dungeon)
8. Ghorovas Rift (add the mini-dungeon adapted from the pocket Edition)
9. Gralea, same but adds the playable section I mentioned above.
The only thing we can be sure about is Tabata confirmed many new monsters in future DLC so that is exciting. I hope some of these pertain to Niflheim wildlife, which are practically reused from Lucis or non-existent save for Malboro