I think that I have an idea for a sequel to FFXV if Square Enix really wants to milk 3 more games out of FFXV without doing a prequel, please bear with me, I would be terrified if they tried this.
This will take place between the time Noct defeats Ardyn in the soul of the star, and Luna and Noctis sitting on the Throne, they could easily say 1000 years past from that so the Chocobros could be dead, as well as any of the important side characters, and main characters, so rebirthing them in a new world as well wouldn't be a stretch.
Have a fal'Cie or Etro reincarnate Noctis and his friends into the world of Versus XV(Think of Eos except more tied to the FNC) as a test to make Noctis into a protector a Eos watching over his people or to find Etro's gate don't worry it will end with the Chocobros defeating the fal'Cie, and going into the afterlife to watch over Eos. So he has no memories of his past life in the original Eos, as he's been reincarnated, but he still has the same personality, same powers just different history(Stella or Luna doesn't matter relationship doesn't matter, his relationship with the Chocobros will stay the same though as well as their roles as a brotherhood just with somewhat different circumstances), he's still the prince, his father is still Regis who is still the King, but the Lucian people worship Etro as their goddess. The king is killed during the negotiations with the Niflhelm Empire, the Chocobros have to fight out of the citadel. They go on a roadtrip to retrieve their stolen Crystal, then it ends in them fighting a fal'Cie, Etro, and Etro enhanced Ardyn, or fal'Cie enhanced Ardyn, or (Oh GOD I Hope this isn't the case(Bhunivelze(I threw up in my mouth a little I hate this villain so much he's so whiny) I'd rather it be his mother honestly or better yet Chaos Garland that would be the tits man.
All this will take place over a trilogy of 3 full 40 hour main story line with several hours of side quests games , and will end with Noctis and Luna on the throne of Lucis watching over Eos as protectors with each of the Chocobros sitting on the other chairs.
Directed by Tetsuya Nomura, I suspect Tabata will be busy with a New IP, or Type-Next.