So just today, Tabata has come out and rubbed the genie lamp summoning Ifrit.
This theory will go into ideas what Ifrit may be and will serve as a base of my new theory.
Ifrit maybe the source of power for the Lucii Ring. Basically the idea is that Ifrit and the first King (not King I, but the first King of the KOTR) made a pact, so that the Lucian Kingdom could gain the power to thwart back the Niflheim troops. But if you look at my previous theory of 'Jinn', of which Ifrit is classified as one, they are very powerful beings said to be immune to normal human weapons. Only magic could subdue them.
So Lucis having gained the power of magic being able to utilize the powers of the crystal, the first King imprisoned Ifrit in fear that their powers might be shared, or Ifrit may turn on them. Even though the first King made a pact with Ifrit, Ifrit was imprisoned. I would think that Ifrit loses his body in the physical realm and his soul is trapped somewhere.
If we look at the central Lucii King inside the ring, we can see that he has horns. The only one who has horns. The Kingsglaive emblem may be a combination of the first King and Ifrit's features.
And of course, having been betrayed by the King, Ifrit's hatred and rage burns against Lucis. But unable to do anything since he cannot exist on the physical realm.
Then comes in Ardyn... who has seen some 'leaks' about being Ifrit. But in this theory, Ardyn is not Ifrit, but a human incarnation/messenger of Ifrit, sent by Ifrit to complete a set of tasks so that Ifrit could make his return. This actually sorta falls into my 'fake' leak the other day.
And we have Ravus, the puppet of Ardyn. With Ravus, he will become the 'vessel' for Ifrit. As Ifrit no longer has a physical shell, Ifrit requires one. And who better than a person who hates Lucis as much as he does.
As to further the idea of Ardyn being a messenger of Ifrit... There is Glauca, not the person inside the armor, but the armor itself. In a previous interview it was said that the armor of Glauca was the pinnacle of Niflheims technology, being made of 'liquid metal'. But why is Glauca the only one wears such armor? Why doesn't other Captains of the military wear similar armor? That's where Ardyn comes in. He who is related to Ifrit, conjured up a armor from 'smokeless flames' (refer to previous theory) to create the armor. And if you look at the armor, it looks almost as if it was smeared by flames. Perhaps Glauca's armor is directly related to Ifrit's powers.
How Ifrit has a connection to the 'plague of the stars' is still a mystery. But I think it has something to do with the balance of light and darkness. The world of EOS maintains on a equal balance of light and darkness, Luna the Oracle helps to maintain that balance. But Ifrit's immense hatred and rage may have disrupted this balance, perhaps Ifrit was once a powerful Archaen. But felt betrayed by EOS who has abandoned his fate to the King of Lucis. Perhaps that's why Noctis has to 'prove' himself worthy that they can be trusted. Maybe the other Archaens knows about what happened to Ifrit and the first King.
Perhaps this plague was the fault of Lucis and that is the mistake they made that Noctis has to redeem.