I adore FFXV, it's more than the sum of its parts. But I think there are simpler, more practical and cost-effective additions they could make to improve the overall game.
I think the most impactful thing they could do would be to flesh out the Noctis-Luna relationship. This could be done with letters from Noctis to Luna and vice-versa -- longer and more detailed letters, fully narrated by them, received each time you camp or stay at a hotel, showing us their headspace, and how close they are. Noctis could grieve his father, lament his responsibilities and find an empathetic listener in Luna, for example. And he would likewise support her as she vents to him. Of course there would be lighthearted humor and slice-of-life stuff too, making them feel like real people.
This would then make later events hit way harder. It would also further develop Noctis, showing the toll everything is taking on him.