Could you give some examples? Cause I can't think of anything in particular.
Firstly is probably just development decisions and design of each. Brotherhood has no english dub, so that right there takes you out of that if you ever plan to watch it (not saying the Japanese dub is bad, but it sucks that they never dubbed it). Also, being an anime, not everyone is into that. The stories here are crucial to how the main characters relate to each other, yet they are told through an anime. They could have benefited from being told as individual flashbacks while you do the Camp tours with the bros, or as a prologue, as well as being able to actually hunt the monster that Noctis was hurt by. Then there's Kingsglaive, another monster. So many things work in connectivity but then there are other odd things that don't. Different voice actors for Ravus, Luna, and Regis, as well as differing designs for Luna and Ravus over their game counterparts. Ravus's motivations....are odd to say the least. In the movie, its about revenge against Lucis for Tenebrae and his mother, but in the game its about protecting his sister. The game unfortunately makes no mention of this after. Disconnect also comes in the form of Glauca. He did so many bad things, is actually a person that knew Noct for many years, yet is not mentioned at all in the main game for all the bad things he did. This is also the game's fault, not just the movie's.
It would be nice if a DLC could just adapt both to their proper forms, with Brotherhood being cutscenes or small gameplay segments and Kingsglaive focusing on Regis, Nyx and Luna, with gameplay elements for each. On the flipside, the main game needs more connections to both, either in dialogue or gameplay.
I'd pay 20 bucks for something like that. The Universe only became a thing because they didn't have time to complete the game like they would have wanted to.