I think the next 4 DLC will not only expand on the story of other characters (Ardyn, Luna, etc), but also expand in the world we play in and how we play it.
Think of it similar to how the Royal Edition handled adding new things to Lucis. I believe episode Ardyn/Luna will be accessible through the main story in fragments. If you play the game from the beginning, these pieces of content could be optional, but slowly distributed to you as you encounter them (changing character perspective; Luna's could be more of an early content thing, Ardyn's would be more late game due to his reveal). When you beat them, you unlock a little more of something new in the main game (be it a new mechanic, magic, area to explore, etc).
Those that have beat the game and want to just experience them whole could potentially access them in post game as a complete story if they weren't completed in the main game.
Things like a Niflheim expansion, Lucis/Accordo expansions, or an Insomnia prologue could be done the same exact way. I think directly adding to both halves of the game to make it a more complete experience is the way to go.
Tabata and crew have said they want to steer clear of separate, plot filling episodes that focus on one event. This is why im confident it will be something along these lines: multi-chapter events that directly affect the main game and new things you receive or get to do (new abilities, magic, areas, side-quests, you name it).
Each individually may not be exactly HUGE in terms of additions (something like Royal Edition sized or a tad larger seems like a feasable thing), but all 4 together quite literally may add another half of a final fatnasy game by the end of it all. It may actually feel "Complete" by then, along with the future free updates, movie, anime, and the rest if the universe.
This is how I personally think the content will be delivered. And if you don't want to play the game over everytime, it will all be available post game.
Bonus: Dub Brotherhood and add them as free menu based episodes we can watch after starting chapter 8. we can unlock them as we unlock everything else in the menu, either through finding something in the world or completing something. Canon-wise, the episodes should take place around there, even though gladio's scars would be missing:/