If I say that a very good job wasn't done in the production and upgrades found in Final Fantasy XV, I would be making myself a horrible lair because of all the series of Final Fantasy games, the best work was evident in the series of FF VX.
But the truth of the matter is that our expectations and desires for more impressive improvement can never be satisfied. We gamers are always going to ask for more. So having said that, I would seek for more upgrade in the music and the graphics of the playable characters in the game. Also still twisting the story plot looks interesting.