These are the known facts, which can be found with little bit of googling and Youtube searching (the official trailers). I reply like this so I can ad some videos here!
The theme of Final Fantasy Versus XIII was darkness. The official quotation used in trailers, from Shakespeare, says “There’s nothing either bad or good, but thinking makes it so”. It was inspired by Hamlet and Romeo & Juliet.
Noctis and Stella were both able to see “The Light”. This was because both of them had had a near-death experience. Everyone with NDE would be able to see this Light yet it was still considered something rare. In the trailer below Noctis meets Stella for the first time and they talk about this Light and their experiences.
Seeing the Light of Etro, the Goddess of Death, gives you power, but in order to harness that power you need to sacrifice someone. Stella and Noctis talk about this in the trailer above.
Noctis had oracle like tendencies as he was able to see in his sleep deaths of other people, yet he was unable to prevents these deaths from happening. At some point, he would lose the ability to tell was he awake or not. We see him being sleepy or resting a lot in the trailers, and FFXV team kept this sleepiness, but made it only because Noctis in FFXV is a lazy young adult and anxious about his future of Lucii Ring bearer. See the trailer below from 2011, where a driver tries to wake Noctis up by calling “Prince Noctis…. Prince Noctis!” (I’m sorry, I couldn’t find English subtitled one, but I’ve seen one).
Insomnia was an eternal dark city, run by Lucis family, which operated like mafia. Noctis was the next heir for the mafia’s throne, but his father Regis wasn’t pleased with Noctis. The mafia like operation is sort of left to FFXV in a way that Noctis works as a part-timer and no one knows him though he’s a crown heir. People living in modern day countries ruled by monarchies (or countries near monarchy countries) can tell that the crown heir simply can’t be anonymous. Everyone knows them, also in neighboring countries. I live in Europe and can recognize multiple rules, crown heirs, princes and princesses of different European monarchies (Sweden’s crown heir Crown Princess Victoria is my fave). However, yakuza and their heirs are unrecognized in Japan.
Lucis family had the only surviving Crystal left and this was important part of the story. I heard the Lucis family misused the Crystal and Stella tried to get the Crystal back.
Lucis family worshipped the Goddess of Death, Etro, who is portrayed in the logo. The logo is not Luna. Hints of this are in FFXV in form of black royal clothes, red bottom of shoes and skull logos.
Stella was a character of her own so Luna isn’t rewritten Stella. Stella and Noctis were supposed to fight each other. They were each other’s love interests due the Romeo & Juliet part, which would make the fight even more dramatic. It’s also hinted in one of the trailers which says “The Star meets The Night” = Stella is Star in Latin, while Noctis is Night in Latin. Stella was from Tenebrae.
Gladio was Noctis’s childhood friend while Prompto was a newcomer. Prompto was a flirty, dangerous playboy and both Gladio and Ignis didn’t like him, because he was bad influence to Noctis. Here’s the original Prompto:
Gladio was going to betray Noctis due Regis’s order. Regis wasn’t happy with Noctis and didn’t want him to get the throne, so he ordered Gladio to kill him. Gladio would eventually die, most likely in the fight between him and Noctis.
Prompto was also going to betray Noctis, as he was Niflheim’s lapdog. His father Verstael was Nilfheim’s mad scientist, who had made Prompto as a human experiment, but abandoned him because he was a failure. Noctis would fight both Prompto and Verstael, ending up killing them both.
Knowing that Noctis could see the deaths of other people he must have known both Gladio and Prompto would die and he wouldn’t be able to save them.
You were able to play as other characters and control them.
The story was going to come for PS3 and it was supposed to be Trilogy.
Here are all the Versus trailers, if you want to see them, starting from 2006 when Versus was announced.
If you’d want to play this game as it was originally supposed to be you can sign petition here.
Tetsuya Nomura himself was personally angry when his project and story got given to another team just to mess it completely over. Only names and some outlooks of characters stayed. 95% was changed. I can feel him; I’d be pissed off if someone assigned my story and my characters to some other group and I would need only watch when everything you have planned and created is destroyed just for money’s sake. We lost Versus only due money talk.
So, I wish for Nomura’s and the characters’ sake we’d get the original Versus, even if it was only 1 game and not trilogy anymore.