Alright, I'll say it right out of the gate. While it does contain narrative, thematic and design elements from its original form as Final Fantasy Versus XIII - the sister project of Final Fantasy XIII and one of the two core projects within the ultimately-failed Fabula Nova Crystallis Final Fantasy subseries (hereafter FNC) - any overt terminology or marketing elements were removed after much internal discussion so it could appeal to as broad an audience as possible. As far as possible, it's its own thing. But that doesn't mean they're not still there. This thread's gonna spell it out. If you want another contributor's theory on the subject, then check out @Ikkin's YouTube video on the Pitioss ruins, which is quite well researched and does a good job deconstructing the narrative of this highly cryptic and symbolic dungeon. Remnants of l'Cie: This piece is the most easily established. l'Cie in XIII and Type-0 are humans chosen by divine powers and given a task, with multiple constrictions involved. In XV Noctis, Lunafreya and Ardyn were all charged by the divine Crystal and Eos' Astrals with a task. Ardyn failed in that task and became a cursed being, while Noctis and Lunafreya succeeded but had to sacrifice everything to achieve it. Tabata even stated in the XV Scenario Ultimania that the theme of FNC of humans burdened by a destiny by the divine Crystal was carried directly into XV. The Messengers, servants and voices of the divine, could also fall into this category, as Gentiana's existence as both Messenger and vessel for the Astral Shiva denote her servitor status and harken back to the burden of the l'Cie. A different l'Cie-related callback is the fact that Noctis enters a sleep-like state within Lucis' Crystal, a state analagous with the recurring FNC concept of Crystal Stasis - a deity or branded human turning to crystal and entering a sleep-like state. Remnants of world structure: This is also easily established. The world of XV appears to have an invisible world that can be interpreted as a dream realm, a divine plain, or simply the afterlife - this point is never made clear, and it could encompass all of these and more. FNC establishes that the world of humans is counterbalanced by the Unseen Realm, the void where the hearts and souls of humanity return when they die or issue from when they are born. There are also individuals (Noctis in XV, Yeul in XIII, Joker and Tiz in Type-0) who are sensitive to this parallel plain, and can even cross into it. Remnants of souls: Again simple to see. Souls in FNC, represented by shards of crystal floating in the air, represent the permanent existence of humanity. In XV, floating crystal fragments continue to be seen, most prominently connected to Noctis acquiring the Royal Arms of previous Lucian kings through the "bonding of souls", and that final scene where Lunafreya and Noctis find their happiness in the afterlife. It's likely this element was just directly lifted, as little seems altered from what the FNC mythos tells us about the nature of souls. Remnants of fal'Cie: This is far more subjective, but can be found. In XV, the world has six god-like beings dubbed the Astrals, who protect the natural world and mostly aid mankind against the Starscourge. But while they could be easily interpreted as the creators of Eos and the be-all-end-all of existence, they refer to a higher power and greater mysteries. This ties in to the next point below. While they generally support humanity rather than manipulating them, the Astrals' status as divine beings created by a higher power closely correlates to the FNC's original concept of the fal'Cie, god-like beings created to enact the will of the god Bhunivelze. A conflict between different fal'Cie factions may also be referenced by the feud within the Astrals between Ifrit and the other five. Remnants of higher beings: This can also prove subjective, but is partially substantiated by Astral dialogue and several incidental details from supplementary material and developer comments. The Astrals are referred to as the "power of the stars", while the Crystal was apparently created by the planet itself, and there are references to the stars as a separate existence. This clear division between the planet, the stars and the Astrals as manifesting the stars' power hints that the Astrals themselves were created by a higher divine power. In FNC, the fal'Cie were created by Bhunivelze and his children Pulse and Lindzei to serve their ends. It seems the concept of higher beings survived alongside the Astrals. The conflict within the Astrals' ranks can also be seen as referencing the original conflict between Bhunivelze and Mwynn/Etro over the fate of humanity, which forms a core part of each FNC game's backbone of lore. Remnants of Chaos: This is perhaps the most subjective, but there are visual references. In FNC, chaos is the primal origin, a force that corrupts anything it touches yet give humans individual will and emotion - it manifests most prominently as either a smoke-like substance or a vast ocean. XV has the Starscourge, a corrupting force that manifests as black vapour; and the ocean into which Lunafreya vanishes as she bids Noctis their final farewell. Note the scene; tendrils of black vapour rise up from the water's depths, and Lunafreya sinks willingly into those depths. The nature of chaos is further reinforced by Ardyn's immortality due to the Starscourge, which parallels other human characters within FNC who are made immortal due to Etro's intervention, although with Ardyn it's more accurate to say he brought it on himself through good intentions rather than through the actions of a sympathetic goddess. Remnants of "the Goddess": This is the most subjective of all. In FNC, and particularly in Versus XIII, the death goddess Etro formed a core part of the lore, directly relating to the whole plot. Ikkin's video has a lot to say about this, but simply to say that the great statue in the Pitioss Ruins could be the last remnant of Etro, although in the context of XV it could easily be one of the "stars" that granted humanity the Crystal - and perhaps even gave the Lucis Caelum line the Royal Arms magic and its role as Eos' saving grace. In addition, recurring symbolism within Pitioss and Lunafreya's final farewell - a powerful female figure sinks into darkness, and in Lunafreya's case passes on her responsibilities to another - mirrors the lore surrounding Etro in addition to other game-specific narrative elements in XIII and Type-0. Of course, there are perhaps twenty other theories that could connect this with another Final Fantasy, or just debunk every theory and its mother. But this isn't a theory, it's just my analysis, which can be taken for what it's worth. EDIT: Many thanks to @Ikkin for pointing these other points of comparison out to me that I'd missed the first time, and can further elaborate on. Those that fit into other main bulletpoitns I've incorporated above. Remnants of Cie'th: In FNC, a l'Cie who fails in their task or is at least deemed to have failed is transformed into some kind of monster - in XIII it's a mindless crystalline monstrosity while in Type-0 it's an amnesiac lunatic. As further described in LR's supplementary material, the Cie'th's curse is basically the chaos within within running wild, twisting the body and mind. Cie'th don't exist as such in XV, but it's more than interesting to not that, while written differently, the Japanese term for Cie'th and Daemons is identical; Shi'gai, a homonym for the Japanese word "corpse". It could be argued that Ardyn is the ultimate Cie'th equivalent, due to his original role and ultimate twisted motivations. Remnants of Versus XIII: While a lot changed according to staff, it's interesting to see what didn't. The basic concept of a fantasy based on reality, the divine clashing with the modern world, and a tragic story persisted throughout. Also notable is the fact that the Latin lyrics for songs - written by Tetsuya Nomura when the game was still officially planted within FNC - were not altered, and continue to vaguely reference the basic FNC elements as applied and adjusted for XV. Remnants of final battle: Now this one is highly subjective, but also highly suggestive. While a final grand battle in a world upset by the story's events is nothing new in RPGs or the Final Fantasy series as a whole, FNC have a particular theme - in a world on the brink of destruction, a final battle determines all. This can even be traced back to the core mythos, where Bhunivelze seeks a final duel with Mwynn for control of all reality, and the opposing forces of Pulse and Lindzei both in service to Bhunivelze's wish. In the XIII trilogy, the "final battle" was represented by the fights against Orphan, Caius Ballad, and Bhunivelze. In Type-0, Tempus Finis is an entire chapter's worth of battles to determine the world's fate. In XV, it's the world plunged into darkness by the Starscourge, and Noctis' final duel with Ardyn for the fate of Eos. Basically, twin forces blessed by the divine battle for the world's fate. While it's been necessarily adjusted in XV, this running motif is noticeable. Remnants of ultimate sacrifice: One of the most common themes within FNC is that of sacrifice to the point of despair, applying to human, l'Cie and deity alike. While this could also apply to some other Final Fantasy titles, it is particularly noticeable within FNC - the l'Cie of XIII, Class Zero in Type-0, and the Lucis Caelum and Nox Fleuret families in XV are all asked to suffer and give their all for the sake of a divine plan, whether the motivation be self-interest or the well-being of humanity. Noctis is literally told by Bahamut that, after others giving their all to save him, he must give his all to save everyone - including his life.