My initial thought was that Aranea is a weird choice for being given her own episode, but the more I think about it, the more I'm coming around to the idea.
I mean, in terms of gameplay mechanics, she's the only guest character who really provides the opportunity for new mechanics. Ravus uses a one-handed sword, Cor uses a katana, and Iris uses hand-to-hand, all of which are options in either the main game or Comrades (or both). While the spear is an available option in the main game, however, Aranea's dragoon abilities offer something new and different. She's also a repeated temporary guest, and it wouldn't be that difficult to allow the player to conscript her into the party and play as her in the post-game.
On the other hand, I'm not sure I'd be satisfied with an episode about Aranea's day-to-day work and decision to leave the empire. The true genius of giving Aranea an episode is that she's perfectly placed to give the player the opportunity to experience the invasion of Tenebrae, which could provide further opportunities to develop Ravus (who we could see captured) and even possibly Luna (whose personal effects we could explore) if done right without retconning untenable amounts of the main game. (Tenebrae's ruins, unlike Insomnia's in the original game, are actually successful at creating a sense of finding oneself left in the aftermath of something tragic that one couldn't prevent.) And given that Episode Aranea would have the same as the director as Episode Ignis, we know he can pull it off.
That could leave the overall roadmap as something like this (with my speculations on the contents, of course):
Episode Ardyn: Ardyn is chosen by the Crystal, decides to go around absorbing daemons instead, gets corrupted, loses the loyalty of his people, and is cast out. Throw in an encounter with Ifrit somewhere to add to the lore regarding the Hexatheon.
Episode Luna: Luna travels around Lucis with Gentiana to forge Covenants with Titan and Ramuh. She's able to convince Titan to give Noct a chance, but Ramuh decides she has to face judgment in Noct's place, and we get a final boss fight with Ramuh. The Sylleblossom quest that allows her to be used in the main game is either part of the DLC or the corresponding free update.
Episode Aranea: Aranea, Biggs, and Wedge travel to Tenebrae to try to help people escape from the approaching darkness. She gets to explore Fenestala Manor for a little while and meets Ravus there before Niflheim attacks. She and Ravus try to protect the people, but Ravus lets himself be captured to give everyone else a chance to escape. Aranea, Biggs, and Wedge manage to drive the remaining Niffs off, but the city is already in ruins.
Complete Edition: Adds onto the main story in the same way as the Royal Edition did, this time expanding the Leviathan setpiece, showing Noct and co. boarding the underground train in Altissia and talking to each other on the way to the western continent, adding the mini-dungeon in Ghorovas Rift, and expanding the outside of Gralea. Also offers natural integration points for all of the DLC episodes and the Brotherhood episodes in the main story.
If they did that, the linear parts of the game would be all but perfect. We'd get to see what Luna had to go through to forge the Covenants that Noct needed to gain the Astrals' support; a properly integrated explanation for Gladio going AWOL in Chapter 7; a more satisfying Leviathan setpiece followed by the ridiculously awesome Episode Ignis setpiece extravaganza (minus the endgame spoilers); a proper transition from Altissia to the western continent; the Chapter 10 downer dungeon and the Chapter 11 setpiece left largely alone, but Chapter 11 expanded out to a reasonable length through Aranea's experience of the devastation of Tenebrae, which in turn supplements Noct's visit in Chapter 12; a mini-dungeon for Shiva to match the mini-dungeon for Titan; a proper integration of Chapter 13:2 into Chapter 13 proper; an opportunity for Prompto to talk to Noctis about everything that happened in Episode Prompto; a full backstory for Ardyn while Noct is in the Crystal; an awesome final boss rush in Chapter 14; and the best ending in all of Final Fantasy. They could even modify the Bahamut fight and the return-to-Insomnia missions from Comrades and add those into the main game to make it even better (with the not-unwelcome consequence that every Astral besides Shiva would get an epic fight in the course of the game!).