Personally, I would also be fine with Episodes Luna and Areana being made into graphic novels(or digital graphic novels), as long as Episode Noctis is still released as DLC for Final Fantasy XV itself.

I can understand Episode Luna being a novel instead of DLC, since Lunafreya isn't much of a fighter, but some plot points from it can still be at least alluded to in Episode Noctis, if it helps tie up loose ends and better flesh out the characters.