I will say I don't think there was much fundamentally wrong with anything in XV, even what I consider the 'bad' plot was mainly bad because they didn't elaborate enough. If someone told me to make changes to XV to put it on par with X at the very least, I wouldn't have to throw much away that's already there, I'd just take the good moments it had and add a hell of a lot more of them

In a perfect world the biggest change I would make would be to make Luna a party member. Have the first big thing you do in the game be to rescue her from the clutches of the empire post-treaty disaster and then she joins up. Having her around in the car and at camp with the bros, being a part of events as they unfolded, then they'd have their Aerith moment when she dies. It would also alleviate my issues with how they represent women in this game in general which is pretty poor in my view.

One thing I thought of earlier when I was considering Aranea (a highlight of the game - again she needed more of an arc though. She felt like Beatrix-lite), which is: why did the empire help Noctis and co get the Mythril for their boat? Maybe I'm mis-remembering, but I'm pretty sure their entire mission was "get some mythril so we can fix the boat". They show up at the mythril cave and the IMPERIAL CHANCELLOR is waiting for them with Aranea the dragoon mercenary. He then helps Noctis and friends get the mythril and then leaves without accomplishing anything else or demanding anything of them, and even gives them a lift out of the area.

Now, considering the empire has just taken over my city and murdered my king and is also trying to kill/capture Noctis... the fact that the chancellor is going out of his way to help me get bits for my boat is unbelievably suspect. The guys should be about 7 MILLION times more suspicious than they are. They say "Oh I don't trust this guy" but they still follow him around everywhere and accept lifts on imperial ships with him and let him send mercs into dungeons with them.