WHAT ARE THE MOTIVATIONS OF THE EMPIRE?
I think Eos was once unified by the King of Lucis. I also think the world went along with it because of the Lucian magic power, aka blessing of the gods. I also assume the world religion was widespread at one point because the role of the Oracle is still recognized as important.
Unfortunately, Ideolas is not religious. Ideolas and his tech guy, Verstael, have found a better use for the remnants of the Big Bang... Magitek batteries. From Kingsglaive, we can see Ideolas has a viewpoint kinda like "survival of the fittest" or "only the powerful should rule." In his mind, if he can defeat the champions of the old religion and their magic power he should rule the world. The virtues of kingly rule be damned! The laws of nature also be damned! Nif uses magitek to control wild beasts and even daemons of the underworld. To Ideolas, even if the gods chose the line of Lucis, he thinks himself resourceful enough to challenge them. Of course what resources can you hope to use if the gods are literally the planet and vice versa... (btw, I'm not including Ardyn here)
Also note the geography of Eos. The ancient Japanese Emperor who commissioned the Kojiki and Nihon Shoki was based in the east of old Japan...the Gate to Yomi was located in the west of old Japan. Lucis is in the East, Niflheim in the west.
WHAT IS THE ROLE OF THE ORACLE?
In Shinto priests and priestesses have a lot of responsibilities. Depending on the rank they have to know a ton of purification rituals, take care of a shrine, or make sure the next emperor ascends to the throne without a hitch. Traditionally, certain families were priests so it was a role passed down for generations. Also women at one time had very specific roles as mediums between men and kami.
I'm thinking the role of XVs Oracle is a little bit of all of that.
Now, what's the point of Shinto? In Christianity, you wanna be saved and avoid hellfire. In Shinto, we may say the goal is to... Not rock the boat. Natural order is an important thread in the practice of Shinto. Kannagara: the way of the kami. Everything in the world pretty much belongs here, and there's no Original Sin that tainted the Grand Plan of things. Evil just exists but folks should pursue virtue and purity.
Shinto priests are experts in purification rituals. I think Luna sees herself as responsible for aiding the world in its own purification. *cough*Plague*cough* of Stars*cough*
Luna like a traditional Miko can speak with the gods and is the last of a long line of priests that doubled as white mages when darkness was in recession.
The rituals for taking the throne as a Emperor in Japan are pretty intensive. You've gotta receive the Royal Artifacts that are taken care of by Priests, visit the Graves of the last three Emperors (maintained by priests) to have a nice chat, take a special bath and eat some super clean rice at the Shrine to Amaterasu to have a nice chat with your great.....great grandmother without a priest, etc.
Noct is going to be on a similar tract, and he's going to need Luna's help. He's going to collect the ring, visit graves of kings, and the resting places of the gods to have some chats/boss fights. Finally, he will ascend to the throne and Godhood(?!)
THE PLAUGE OF STARS AND HUBRIS
So I've pointed out what could be some strong parallels between XV Lore and Japanese Tradition, but now I'm just going to riff through my tinfoil hat ideas.
FFXV's main conflict is going to have a strong classic Japanese feel to it. The "big bad" may not a evil wizard like Zeromus or Exdeath, it might not be a dejected Demi-God like Kefka or Sephiroth, it might just be a force of Nature. I'm thinking like Godzilla, like Cloud of Darkness if it were powered by the worlds agnst. I could see some strong environmental themes showing up because anytime the gods are brought up we're talking about the planet itself. So the Plague of Stars may be the result of the Nifs use of magitek, or it could be like a wave of darkness produced by the fear of people in the crossfire of the war.
Or one twist, the Plague of Stars could be a void left by the gods whose powers are waning because of a lack of faith among the people. So, when Tabata mentioned that XV lore has a Japanese base he also said there was a western bit too. I think they are going to pull from Judaism.
NOCT IS THE MESSIAH
There is a certain tribal & political feel to the Jewish and Japanese religions with certain families being selected by the gods to care for and protect a people and country, but what happens if that country is lost?
There's a ton of stories in the Bible about the Israelites in diaspora and from there a messianic tradition grew. The messiah's like this champion who can unite his people because he's from the royal family and is equipped to judge the enemies of his god.
THE KING OF LIGHT(?)
I think the Lucian prophecy is about a time when the gods will leave/the gods will awaken and judge the world, going first to their handpicked representative; if the king fails the gods ditch the world and eternal darkness is all that is left. The blessing could have been contingent on this regular checkup on creation. Another twist. If the King fails judgement, he becomes the executioner. Like a vengeful Cieth...
The Lucians/Luci/L'cie worship a God of death(in vXIII), wear black, and were gifted traditional black magic. (I think the skull emblem is still in the game.) I'm thinking about a biblical story in Exodus where god's people marked their doors with sacrificial blood because their god was about to kill the first born of every family without the mark, as retribution for using them as slaves. The Lucian power can be like that of the grim reaper, if necessary. Maybe Noct will be tempted to use that power to avenge his kingdom(like Ashe in XII). Ardyn may want to harness this power or could be the only one to recognize the threat it presents.
One more thing, the first mention of impurity in the Kojiki is death. Izanagi goes to visit his dead wife Izanami in Yomi the land of the dead.[Btw she died while giving birth to a fire Kami like Ifrit(?) who is missing from the Genesis mural(?)] Anyway, Izanagi freaks out that Izanami is all zombified so he bathes so vigorously he births three gods including Amaterasu. [Maybe Noct will enter XVs Yomi/Unseen Realm/Land of Dreams to find a dead Luna(?!)]
If Lucis still has ties to "Death" in XV, how could they be "chosen" by a possible analogue to Amaterasu, a light goddess? Death is very impure. Maybe that is why the King must always keep the Oracle close, to right the ship at the time of need.
I'm also thinking the Luci (dead kings) are somehow contributing to the Plague. The Luci as immortals may go against nature by simply existing. Noct's ancestors may be harnessing the blessing improperly. I think of the Plague as a force similar to Chaos in FFXIII:LR, a darkness creeping in from the realm of the dead. It starts as a drip, then a flood as the Archaeans awaken and manifest. Metaphysically, the Archaeans' powers exist in the world as the elements, but too much of their power when concentrated shake creation. The connection of the Luci to that power may be what is powering the connection between realms.
The resolution in XV'll probably be classic FNC, with Noct saying buh-bye to the gods and magic, saying he can rule his kingdom with the support of his chocobros alone.