I had someone ask me once what order to play the Episodes during a playthrough. I think E.Ignis could be implemented during Chapter 14 anytime before the Ifrit fight. For the Royal Edition I would say after Cerberus.
The thing is, if they implement the episodes, they should be edited in a way that feels cohesive with the narrative.
I think i already said this, but in my mind the best way to implement Episode Ignis is start it when Luna summons Titan, just before the CGI of Luna giving his farewell to Noctis.
The Episode would play as normal till you get to the altar with Ravus (ending of chapter 2), at this moment is when the CGI should play, to making us feel like we're to late, and going back to Ignis to find Luna dead with Noctis at her side.
Then chapter 3 should be butchered. With us not seeing or hearing what Prinna shows to Ignis, and without appearing a menu where we should select if we want to take the canon route or the Verse 2.
Once Ignis says "please, forgive me." the story should pass to Noctis again, waking in Altissia, but before them leaving the place,the scene where Ignis comments about giving up should be played.
At last at Chapter 14 after the last campsite, the ending scene of the episode should be played. (In my first playthrough i felt like there should happen something, to make the pace less rushed).
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Sorry for publishing 3 posts in a row, using forums with the phone is a pain in the ass.
I think if the original episodes are implemented in the narrative, then the new 3 would feel combuluted in the narrative.
Knowing BD2 is planning an alternative ending. Maybe they're creating an entire new second part of the game for a NG+ route taking the events of Verse 2 as a start.
If they make the content as something postgame we could get to see the Episode Ardyn, Luna, Aranea, as parallels of chapter 10 to 13. Retaking the control of Noctis for the last act.
Even as an alternative story, the lore showed about Ardyn, Luna, Aranea and Ravus could complement the main story.