First of all, Prompto's not an MT. That's just some localization bullshit. He's only from Niflheim, not a Magitek Trooper at all, most likely just on his way to becoming one before he left the empire.
That's one thing I'd really like for them to have done better, though: Characters properly processing information and reacting to them, etc. Noctis and co were distraught by the events of Kingsglaive, but then right after that you're all cheery and shit. Gladio and Iris don't mention their dad at all, not even a hint of him. There should have been scenes for these things,
such as a montage depicting moments like these, especially the revelation that Prompto's from Niflheim. Although, to be fair, it makes sense that they didn't spend time to dwell on that revelation as they were in a hurry to stop Ardyn and whatnot. What they should have done was flesh out Chapter 14 and have Noctis catch up with his best friends and the things they've learned. It should have been included in a campfire scene set prior to the mid-credits (or was it post?) one.
As much as I like the idea of "opening" the game with the broken down Regalia, I would have preferred it if they stuck to Versus' plan of the Insomnia invasion being the opening. They could still have the Regalia break down the same way it did in the game, but the opening should have been a playable Kingsglaive. Have us play as Nyx, playing through the events of the whole movie and then some. It would be like Alexandria from IX in a way, as well as XII's opening scene with Rex. Insomnia would be explorable and not wasted at all, in addition to having players have a proper emotional connection with the Crown City. THEN you shift to Noctis and gang in Leide.
I agree as well that the open world setting hurts the storytelling. It's what caused one of the complaints I mentioned, about how right after Kingsglaive's events, the gang practically goes back to fun road trip mode right after. The temptation to play all these side quests and explore the world prevented the story from flowing naturally for me. There was a huge disconnect that made each segment not have as much punch as they should have. I do love the open world, and it totally makes sense given the context, but I think I might have preferred it if we were given large open areas instead, kind of like Dishonored's.
Cor should not have been wasted. He should have stuck around much longer and played a bigger role. If you ask me, it's pretty irresponsible of him to just leave the new king like that. I get that you have to trust him and he has to learn on his own as well, but Jesus, the Empire practically just nuked your capital, and killed the previous king! Have him stay with the party some more, scenes of him telling us about Insomnia and Regis, etc. Plus, I don't know, I was kind of hoping there was something big behind his nickname, "The Immortal." Would've been cool.
Luna was wasted too, I feel, but I wouldn't change her fate as much as I wish she didn't die. At first, I thought that it was just awful to kill her off just as we finally caught up with her, but doesn't that kind of just emphasize the tragedy more? In a way, I think it's somewhat brilliant, the way they ripped her away from us just as we finally reached her. It's cruel and effective. Well, it would have been much more effective if we got to see and learn more of Luna. It would not have been nearly as effective if, say, she joined the party and instead died from all the covenants she was forming.
Verstael and Iedolas were totally shat on. That should be fixed. The last they could have done for the freaking emperor was give us a cutscene for Foras when we encounter him and later when we defeat him. I mean shit, some people didn't even know who he was.
As much as I love Aranea (thank fuck she got a lot of screentime and whatnot, I thought it would have been the opposite), I wouldn't want her to be a permanent party member. A guest party member the way she is makes more sense, although I would like it if they could make that a little more controlled and for longer periods.
Lastly, yeah, Chapter 14 should have been explorable. The World in Ruins is such an exciting thing, and it was just awful of them to give us all these details about the world and characters and only have us be in Hammerhead and Galdin. It should have been explorable, complete with side quests. Speaking of which, they could have been used as a means of catching the player up with each of the three guys, etc. Oh, and Prompto's goatee has to go. Shave that shit, son.