Note: Be warned that there will be uncensored spoilers in this opinion piece.
Second Note: This article contains my own expositions, interpretations and theories about the XIII games, and it is every reader's prerogative to agree or disagree as they choose.
Final Fantasy XIII, and its sequels XIII-2 and Lightning Returns, have divided the fanbase mostly because of their gameplay and characters. The story has also come in for some criticism, but from a purely academic standpoint, it's fascinating to see how the rest of the series and the real world has influenced these titles. I don't mean the vague or explicit allusions that are made either deliberately or unconsciously, but instead how many plot elements can be traced and paralleled to both other entries in the series and real-world mythologies.
Magic, War and CrystalsEdit
The story, setting and universal structure of these games are their own in a way, but also share many similarities with other games in the franchise. Whether this is accidental, unconscious or deliberate is debatable, but these similarities are very entertaining, and might even give a hint as to where the series might go in the future.
One of the biggest pieces of story overlap is with Final Fantasy VI. In VI, the world is in a semi-industrial stage of development, on the cusp between its modern future and its magic-driven past. 1000 years before, there was a war between deities over domination of the world, and their war gave birth to the Espers. Upon making peace, the deities and their Esper soldiers retreated from the world, making magic a rarity and leaving humanity to take a technological path. Those who wield magic are alternately revered and feared. In the time of the story, magic is one again harness and eventually used to bring ruination to the world. This world, on the brink of destruction, is finally saved and allowed to recover from its past upheavals. XIII and its sequels follow a similar pattern: one world scarred and another world chained by ancient rivalry between deities, people who wield magic and are thus both revered and feared for it, a great catastrophe which sends the world into ruin, and a second chance for a peaceful life as the old world fades for good and the new world rises. The cast of VI also have parallels among the cast of XIII. The parallels are both obvious and subtle: Celes/Lightning, Terra/Serah, Locke/Snow. Bhunivelze and Kefka can also be seen as similar: both are deity figures, wear purple robes, have angelic wings, wield great and deadly power, have cult-like followings and each, at some point, wishes to destroy the world. However, their motives are universes apart: Kefka is fueled by an extreme form of nihilism, while Bhunivelze is fueled by his longing to live and his inability to understand either Chaos or the human soul.
Final Fantasy XII also shares a few elements with the XIII games. In this world, crystals are given to the mortal world by the Occuria, a race of semi-divine beings who use the people of the world as playthings and manipulate events as if they were making sketches, throwing them away and drawing them anew. It also features characters who wish to break free of the machinations of the Occuria, including Cidolfus Demen Bunansa, Vayne Solidor, and eventually Ashelia B'nargin Dalmasca. The XIII games also feature deities who used crystals to control the mortal world and to whom the lives of their subjects were a nothing, and a whole cast of characters who wished for humans to control their own fate, even if it meant turning the world upside-down. As to characters, the link between Ashe in XII and Cid in XIII defying their roles is glaringly obvious.
The role of crystals in this world may be obscure at times. In the world of XIII, crystals form the cores of the fal'Cie, are their means of forcing their will on the l'Cie they choose, are linked to Bhunivelze, are the forms human souls take to pass into Valhalla, and are generally linked to the very existence of the universe. Crystals as the core of the universe, or at least an important part of it, is a plot motif that can be found in the original Final Fantasy, in III, IV, IX, XI, XII and XIV, and in many others. Whereas crystals in those other settings were mostly benevolent, in the XIII universe, they were predominantly malevolent or at least the byproduct of malicious intentions. In this respect, they can be compared to the crystal pillar in Final Fantasy VIII, which allows monsters to pass to the world below. However, at the very end, it was a crystal that gave birth to a new world free of gods, so it can be said that they weren't all bad taken all in all.
The Myth of the Real WorldEdit
Naturally, all aspects of the Final Fantasy series owe something to the real world, be it history, mythology or common story types. It was established in the first game's Ultimania Omega that the mutli-ethnic feel of the United States was meant to be emulated in the setting and characters of XIII, while that rather jarring final sequence in Lightning Returns has all too many parallels with the real world: would have put in more odd features and used Pulsian writing myself, but there... The mythical elements and references in XIII and its sequels are just as fascinating as its connections to the series, taking in cultures from east and west.
The main mythos behind the games, and a few others as the indoctrinated will know, is based primarily on the Japanese creation myth, with a few poetic touches and other elements thrown in for good measure. In the Japanese creation myth, a divine couple, Izanagi and Izanami, sired the main generation of deities. While the myth also provides a convenient excuse for gender inequality in Japan, it also tells of how Izanami died during the traumatic birth of the fire god and was drawn into the invisible Land of Darkness, realm of the dead. Izanagi wished to retrieve his loved one, and so went to the Land of Darkness. But when he arrived, Izanami was irrevocably bound to the underworld, and he fled in fear. She was consumed by rage, and Izanagi permanently blocked the way to the Land of Darkness, separating the two worlds and the couple forever more. After purifying himself, Izanagi created many new deities to rule over the heavens, land and sea. One rebelled and was cast out, and so the deity retreated from the world forever more.
Within this myth are the key ingredients of the XIII mythos: an invisible realm where the souls of the dead gather, a dead goddess resting within it, a god of the living who feared death even in his dearest love, a set of lower deities charged with ruling over the world, one deity that was cast out, and a father deity who vanished from the world. With a little imaginative interpretation and extra Final Fantasy-esque narrative, it's very easy to make the connection.
The other mythos that shows up in a big way in the XIII series is Norse mythology. This has a mass of competing deities that is essentially one huge extended family. It also has a timeless underworld ruled over by a despised goddess who was cast out by the main deities because of her origins (in this case Hel, hag-daughter of the fire god Loki). It also features a primordial nothingness from which came the essentials for the creation of the world we know. The Norse myth revolves around a destined apocalypse called Ragnarok, which is where all the deities have a final free-for-all, nearly all dying, and the world is consumed by the sea. After this, a new world is born, humanity surviving to tell future generations.
Ragnarok features highly in XIII as the monster said to bring down Cocoon onto Gran Pulse, triggering the End of Days. The myth of the humans who would repopulate the world after Ragnarok were also referenced in Fang and Vanille, although it would have made more sense if Fang had stayed a male character. While that particular event is stalled in a way, Ragnarok did end up being enacted at the end of Lightning Returns, when Lightning led an assault on and defeated Bhunivelze. With him gone, all the deities were either dead or made impotent (Pulse and Lindzei became his weapon and were presumably destroyed during the fight, and we all know what happened to Etro and Mwynn) and the world could begin anew without the old gods and with humanity free to forge its own path.
A less-obvious influence is Greek mythology. The entire Fabula Nova Crystallis series has been compared to it by Hajime Tabata, but it goes deeper than that. The deities of Fabula Nova Crystallis are similar to the Titans and Olympian gods of Greek mythology: flighty, in the latter's case childish, mostly anthropomorphic representations of natural forces humanity can't understand. It may not be much of a connection, but the fal'Cie demonstrate it very strongly. The main Fabula Nova Crystallis deities are more like the abstract forces that spawned the Olympians and Titans: Gaia, Uranus and all the others spawned by the formless, genderless Chaos.
A final, and equally on-the-edge influence is Gnosticism. For the uninitiated, it is a doctrine of thought supposedly co-founded by Simon Magus and practiced by the famous early Christian theologian Valentinus. It features a multi-named creator deity is far more complex and glorious than anything humans can portray, and holds themes of dualism and monism at its core. The deities of the mythos can be compared to the framework of the Gnostic tradition: there is the single creator deity (Mwynn) who ends up spawning by its own will or not lesser beings who wield god-like power, but are not true gods (Bhunivelze, Pulse, Lindzei, Etro, the fal'Cie). Bhunivelze and Pulse can be most clearly linked to the lesser beings, or Demiurges: the original Demiurges are crafters who maintain the physical world, but are also malevolent because of their very existence. They, like Bhunivelze, also cannot see or comprehend aspects of the spiritual world.
And to the future...Edit
Right, now we get into the realm of terrifying speculation. Admittedly, the XIII games are love-them-or-hate-them affairs, but the team behind them, through their wish to expand the world and character stories, created the first true trilogy in the franchise history. Now it has been done, it could well happen again, albeit in a less chaotic fashion. Square Enix has learned from its mistakes, and will probably take their lessons in account if they should ever decide to embark on a decade-long three-game trip into one of their worlds. Also, they have learned that complex mythologies can be a double-edged sword: it can draw people in, but if you don't give enough context, it can prove more confusing than entertaining. It is also a good example of taking myths from the homeland and giving them that extra spin: something that was very effective in Final Fantasy X, but has been sadly underused since then. In a plot-centered thought, this world has managed to work really well with crystals being more linked to fiddlesom, meddling gods than as the lifeforce of the universe. Who knows, they might decide to take that angle again.
So, who knows, we may well see another trilogy, we may see a truly open world mainline Final Fantasy, we may see another game with an incredibly detailed and involving mythology, we may see games within the franchise with even more parallels with the real world. Whether any of this happens or not, or whether something totally unexpected happens, we must just wait and see what the future will bring.