I was thinking they could insert either a 30minute CG cutscene of the invasion when the bros go to sleep after doing Dino's quest. Or alternatively they could end chapter 1 there and insert a new chapter 2 that would be a playable Nyx or Regis in Insomnia.
I would be the first one to buy a playable Kingsglaive. But If SE do this, people will have no reason to buy/see the movie and that would be shooting at their feet.
I also don't think that much time should be dedicated in a single cutscene, it kills the pace (the longest cutscene in the game is the ending+credits and it's only 15 minutes long).
The best thing i can think of is seeing the last day at Insomnia from Noctis perspective and meeting all characters that appear at the movie.
But it's still hard to place it in the game in a way it fits the narrative and i don't have the feeling of my idea being the best solution.
Edit:
One of the things i once though it would improve the relationship of Noctis and Luna would be having the posibility to read 3 or 4 pages of Luna's diary every time Umbra gives it to Noct, hearing a voice over from her and him to emphatise emotion and making it more comfortable to look.
In those messages we could read entries of them as kids and other of them as adults. Being one of them a message telling how much she liked when her mother played the piano, with an answer from Noctis saying he will learn and play for her.
Then, once chapter 12 is improved (let's hope it happens) you go inside Tenebrae and Noctis comments to Ignis there's a piano.
In this scene we can see how Ignis starts playing has trouble with the two hands because of the lack of visual aid, so Noctis sits at his side and plays half of the keyboard.
While the music plays, Noctis tells why he asked Iggie to teach him and how much he practiced to play for Luna at the wedding.
While they play we can se through a window reflection how Luna is looking the scene(like it happened in Omen trailer) but when Noctis looks there, there's nothing.
I think the song that should be played here is Noctis theme (the post game menu song), this way we can have even more feels when we see them together in the logo.
Damn, i touched my own feelings with this );