I just honestly don't see Luna being playable, is all.
I think she is more of a support type character, so having her playable would be kind of weird but if they made Ravus playable and her as his support that i can see.
I'm inclined to agree, but it definitely should be wasted on Ravus, etc. What they should dedicate a DLC to is sprucing up the first half of the game with a lot more story and activities to do. Maybe even open up Northern Lucis as an area to explore.
Basically my dream DLC's go like this:
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. 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.
THIS, would make the game ultimate for me. I'm confident that at least 2 of these things will happen. There's not much else they could add that hasn't happened yet or isn't important to the main story.