I'm getting Big Bang vibes from the artwork. I am 100% convinced that the DLC was supposed to be part of the story, but the challenges of making an open world and time spent on Leviathan and demos harmed their ability to flesh out the game. It also lends ear to the belief that they are in fact using more of the Versus story.
It really bites. They called it an "alternate grand finale", but even Nomura said that the story is so dense, that cutting it would make it lose elements, and remarked that there will be more than one climax. This was way back in the day, and it appears as if the bulk of the story regarding the divine and gods was delegated to DLC as they realized only one fourth of the story, which was the road trip. Because they couldn't have the invasion, they couldn't have Niflheim as a true physical threat; because they couldn't have Luna, having the dream sequences wouldn't have made sense.
Because they couldn't have Ardyn's full story, he was turned into a purely mysterious man, and because Aranea's story was cut, we never got to see the fall of the Empire. We focused on one timeline, and merely the Point A to Point B part of the story. Even Ignis delved into it, appearing as if they were teasing the Versus for the Dawn of the Future. They knew more than we did and had plans, scripts, story and other stuff that we could only dream of and knew how to trim elements from the base game to make what was in the base game as strong as possible. Ergo, we got a bro trip. We lost out on the why of the Crystal, the gods, the "most important goddess", Luna as an ally and more. The old scripts that had more foreshadowing to Ardyn's true nature were changed because they couldn't have Ardyn's betrayal. Kingsglaive itself was made to feature the invasion but rather than give us Noctis which would make people feel cheated it wasn't in the game (which happened anyways), they tried to interject original characters. Some scenes with Nyx very much seem to be like they once belonged to Noctis, like when Noct is watching the Niff ships arriving in Versus while sitting in a car, which we see Nyx doing in his apartment in the movie (which helps us establish when exactly Noctis leaves the city in Kingsglaive).
There's so much to digest here, and people talk about how it's too different, this all seems very in line with the FNC lore established. I really want to do a big chart or maybe even a video on how the lore translated from Versus to now. The main game itself isn't even the main timeline, as that's merely Regis's ideal version of events (Noctis gathers arms, gods power, and he and Luna fulfill their calling), with Ignis's Verse 2 being another (my headcanon is that he's dreaming about what could have been after the events in Altissia, and wakes up having dreamed an ideal version of events) which Noctis sees in the cut Forest of Memories (where he is reflecting in his dreams).
Even the way Luna is changed after death, it's in the logo itself; Luna as goddess is very much not a far cry from how certain characters in XIII are treated, like Yeul. Like in XIII-2, Luna and Etro dying is what causes the Chaos/Starscourge to take over. The idea that the main plot of XV being canon yet still existing alongside this new alternate finale is like out of XIII-2, where they attempt to change the future multiple times, having different results until they get their ideal one. Lightning Returns had a big bang event where they made a new world free of the gods; does this not sound exactly like how the final battle was supposed to go? Insomnia being raised into the sky like the big bang artwork, which seemingly has the gods, daemons and other deities being whisked away to elsewhere while the humans forge a new world free from the gods?
FFXV's DLC was absolutely going to be the third sequel to Nomura's game. I'm of the opinion that Season Pass 1 and base XV made up Nomura's first two sequels, and the third game would have been the versus, and Tabata was merely seeing the task out himself. In the age of DLC and season passes, I can understand the logic that led to them refitting Nomura's sequel ideas into DLC, and wanting to build up a single game. FFXV may have been the largest Final Fantasy story that we'll never see. Let's hope one day we get a new artbook with all concept art.