Well since I put the effort in to it previously, here, I tried to be as logical as possible with what they could improve on with DLC:
1. Ep. Ardyn focusing on his time rising to chancellor, reviving Ifrit, fighting Shiva, and dealing with his past memories on how he was betrayed. Covers a small area of Niflheim and allows Ardyn's car that's "been with him for a long time" as well as potentially summoning ifrit.
2. The first of a 2 part DLC that first focuses on adding more story to Lucis (Chapters 1-8) and Accordo (Chapter 9). This would give more story to Cor, Aranea, Iris, Ravus, Luna, and bits more of the Empire on their happenings during this time period. Every chapter could use more scenes or more story based quests. No need for a new, focused character episode. Make it more about the main game. Add Northern Lucis and a few more islands of Accordo to explore and make these areas more meaningful. Revamp the Leviathan fight.
3. The 2nd part of this DLC that expands on Niflheim as I said above. Chapter 10 focuses on Altissia to Succarpe, boarding the training, opening up Cartanica and going to the Dungeon to get the Katana. Chapter 11 focuses on stopping in Eusciello to get more lore on the Desert/Snow phenomenon and explore that factory town, as well as actually destroying that Niflheim base in Piztala before Prompto's incident happens. Chapter 12 would be stopping at Pagla (there's literally a train stop for it already if you start chapter 12 you can see it) to let some people off there first and deal with figuring out what happened to prompto. Then go to Tenebrae, explore a dungeon or a decent area and then Shiva and do the same thing. Chapter 13 in Gralea could be the same thing but before you get the Noctis/Bahamut scene, it focuses on the Bros trying to escape Gralea from Ardyn through parts of the city. This DLC would help characterize Niflheim so much and actually make the Train section feel like an actual trip. Gives more context and story to niflheim and honestly Regis, who's kind of important during these chapters
4. Lastly, a prologue DLC that is more passive and lets us explore some of insomnia and Nocts life there. Rounds up the game to make it feel like one complete package along with the other FFXV universe content.
5. Comrades should be updating during most of this hopefully, taking advantage of (what I hope to be) the WoR and older characters like Iris/Cindy/Aranea.
On that character episodes:
I don't mind things like this, but I DO hope that character episodes specifically end with Ardyn. If they do have to make a Luna one, it is a 5th DLC and not one of the 4 (I know that is unrealistic to expect, but still).
Id rather the last 3 actually improve areas of the game rather than being specific points in time that focus on a singular event.
It actually goes along with Tabata said in his previous interview from Dangeki, that People really aren't looking to fill plot holes like the previous episodes did, but he still wants to "complete FFXV" in a way that is meaningful. He said to not expect the same format as the previous DLC and to expect a new method of delivery.
He also said episode Ardyn will pioneer that idea. To me that sounds like Ardyn won't be a singular event selected from the main menu, but rather something integrated to points in the story (whether or not that is easy to do will remain to be seen, or how difficult it would be to keep his identity a secret until the big reveal).
This is why the other DLC may not be character focused, but story focused on different areas. Expanding Niflheim may coordinate that idea perfectly. Expanding Lucis's story may do that as well. Making a prologue that fits well into the story could also do that.
But I don't see an episode Ravus/Cor/Aranea/Iris really doing that justice (the last 3 should be playable characters eventually in comrades).
I agree that Ravus and Luna should have more interaction and story scenes though.
Tldr: We need reasons to play the game over from the beginning and enjoy a more complete story. Solving Niflheims problems doesn't solve Lucis's problems and vice versa. The above alleviate problems of the games structure. Ardyn's focuses on the villain (just more of him is fantastic so it's okay), the next 2 focus on the core game, and the last one, as with the Royal Edition focusing on the ending, could focus on the beginning.