http://www.dualshockers.com/final-fantasy-xv-no-plans-dlc-main-game/
He also talked about this at the fan event at gamescom last week:
When Square Enix released Final Fantasy XVs bro-specicfic story DLCs Episode Gladiolus and Episode Prompto, many were surprised to see that they were fairly stand-alone experiences, that couldnt be played as a chapter integrated in the games main story.
DualShockers had a chat with Game Director Hajime Tabata, and we asked him whether there is a possibility for those stories to be ever integrated within the main storyline as their own chapter, but we hear that there is no such plan at the moment. According to Tabata-san, the game released last year is what is considered the main story, while the character DLCs are a supplementary experience.
He also talked about this at the fan event at gamescom last week:
Audience: Whats it going to be like, when the game is completed? Everything released, no more DLCs, when its all complete, will there be a way to play the entire game in like, one go? Like, as of now you always have to play a bit, then, when Episode Gladio happens, you have to go back to the menu screen, select the episode, play it, then go back to the main game, as with all the other bonus content. Will there be a finalized version where everything is just put together properly so you can play it all in one go?
FFXV team: To be perfectly honest, we really didnt think about that yet .I really like the idea! The thing is, that would be really difficult to do, implement all the segments, and I think the changes we would have to make in the programming might turn out to be too problematic we will definitely think about whether we might find a solution to that. Thank you!