So as shown in the Dawn trailer and heavily hinted at in Kingsglaive, both Noctis and Luna have some kind of great destiny they must fulfill, but oddly enough while Luna and Regis are aware of this, Noctis isn't. So what could these destinies be?
As I stated in my Cosmology of FFXV thread I theorized that Noctis and Luna were given their destinies when they had a near death experience 15 years ago. It's probable that the gods decided to give them a second chance at life but in return they were given a task to fulfill. Those who have been given a second chance by the gods after a near death experience and a task to fulfill are FFXV's equivalent of the l'Cie. Now for Noctis, I believe that his destiny is to be the one who will ultimately defeat Chaos (the name we've given to the demonic figure in chapter 0) once and for all. But will he survive this apocalyptic battle? We don't know for sure.
As for Luna, I believe her role is going to be a messianic role that will ultimately require her sacrifice. If the Plague of the Stars is any indication, and if the goddess Eos is indeed the sun, then its possible that she isn't just sleeping, she's dying. This once again parallels the role that Etro had in FNC, a dying goddess who will need a successor. Luna will eventually have to become that successor, which means that she will have to ascend to godhood but ultimately lose her physical form.
One thing I've noticed from the behavior of the Archaeans and the Lucii is that the gods seem either indifferent to humans or outright callous. Why? As I stated in my cosmology thread, it's likely that the gods gave humans the crystals as a way to fight back against the dark god and the plague as well as to seal Chaos, and also symbolized a pact between the gods and humanity. Perhaps due to Chaos manipulating humanity's dark side and making them war over the crystals, the gods feel like mankind is no longer worthy of their protection or consideration, and at this point are only using them because they're vital in the war against Chaos. The journey Noctis and Luna will make won't serve to just save the world but perhaps redeem humanity.
Speaking of the relationship between the gods and man, where I believe Niflheim and Ardyn will fit into all of this is that of a misguided empire seeking to liberate humanity from the clutches of divinity. Ardyn is quite obviously a lot older than he appears and knows a lot more than he seems. I don't personally believe Ardyn is a servant of Chaos or an Archaean however. If the l'Cie comparison still holds, those chosen by the gods can be put into a sleep to be awakened years or centuries later, much like the crystal stasis of FFXIII and Type-0. Which would make sense if the possible leaks of the ending taking place 10 years later and Noctis and company going into a sleep turns out to be true. If this is the case then I believe Ardyn is someone who lived hundreds, maybe thousands of years ago, and also went into a near death experience and was given a destiny by the gods. However at some point he resented this role he was given and sought to defy his fate. He then went into a sleep somehow and when he awakened, he came to Niflheim, a nation founded without any crystals and solely through mankind's ingenuity. We know that Ardyn was the one responsible for Niflheim's advancements in Magitek. Perhaps he gave them ancient knowledge which would allow them to infuse magic with technology, magic that would be free from the control of the gods. Ardyn told the emperor about his past and the truth about the gods and the crystals and Chaos. Thus, Niflheim is attempting to unite humanity under a single nation free from the gods' control, and Ardyn is seeking to somehow cause a conflict between Chaos and the Archaeans which would destroy each other once and for all, allowing humanity to live free and in control of their own destiny.
And then there's Ravus, a man who lost his nation and his family, and bears a deep hatred for both Niflheim and Lucis and wants to destroy them both. He sought to seize the power of the Ring of the Lucii for himself but it rejected him, nearly killing him. In his near death state, his hatred and anger and strong desire for power attracted the attention of another deity. Unlike Noctis, Luna, and Ardyn, who were chosen by the gods of light, he was given a second chance by Chaos. He willingly becomes an agent of Chaos, believing that with his power he will be able to overthrow the gods and become a god himself, allowing him to reshape the world as he sees fit. His bitterness and misanthropy will drive him to want to reshape the world to his own liking as he sees the current world as unworthy of existing and miserable. Ultimately he will betray Ardyn and the empire to do this.