Ok, let's say that the theory I am about to expose is a crazy one since there aren't clear proofs to confirm it, but I want to share it anyway with you all in order to see if someone else had my same thoughts.

Here we go: We know that this war has been very bloody and that it involved the whole world and thus all the people who inhabitate it (as if it isn't obvious. XD), driving the Nirvit to create Nirvana, a magic which in their opinion could have stopped the war by converting the darkness in the souls of people in light, with however the results that we all know.
Now let us pass to the factions who fought against each other in this war; In fact I think that those two were guided respectively by the Dragon King and the Demon King (yeah pretty clichè. XD)
Obviously the main soldiers of both sides were the lesser dragons and demons, but beetween them there were also humans, who according to their good or evil nature, decided to join one of the two parties.
The first ones became Dragon Slayers while the other ones became powerful magicians by receiving the great power of the Demon King, which in my opinion is a part (Darkness-Hatred) of the so-called "One Magic" (since the other half could represent the "Light-Love" Lucy spoke about).
Beetween the DS only three, despite being young, became prominent figures in the Dragons' army...and guess about who I am talking? Yes, Natsu, Gajeel and Wendy.
Instead in the Demons' army the only human who showed an incredible magical power was non other than the black mage Zeref himself, who had the ability to summon an infinite number of demons, thus replenishing continously the troops of the demons.
At some point, after fierce battles, Natsu was somehow able to make Zeref understand the importance of life and thus the latter decided to live a secluded life on Tenro island and wait for the moment in which Natsu could have destroy him, something that he seems to desire in order to atone for his sins (at least in my opinion).
Deprived of their infinite supply of troops the demons' army soon collapsed and the Demon King was eventually sealed away underground in a far away land, which no one could ever reach, because almost inaccessible and protected by very strong magical barriers.
But the seal could be broken by strong magicians who devoted themeselves to the dark side of
"The One Magic", even though this process would have taken many years to work or even centuries.
In order to prevent this the dragons decided to put in a sort of hibernation Natsu and the others, since they showed great capabilities and heart; Thus they would have been reawakened when the time was right (the actual timeline, so that, even if still young, they could improve further their abilities).
In order to ensure the safety of the seal, the other humans decided to create organizations that in the future would have gathered and trained humans in the use of magic, which could enable them to fight back the demons: the Light Guilds.
Unfortunately the same example was followed by the few survivors of the demons' army, who started to create the Dark Guilds, whose leading one was the strongest: Tartaros.
I think that this is the only Dark Guild that treasure its primary objective: resurrecting the Demon King and this is why someday our heroes of FT will have to fight against it to prevent this.
Anyway In the recent chapters I think that the future we have seen briefly, was caused by the clash beetween dragons and demons, and it provoked the deaths of many people, as Levy (presumibly in the future) said.
In little words I think that the Eclipse plan is just one of the prerequisite to dissolve the seal that is binding the Demon King, since opening it would spread "The Dark One Magic" in the whole world and from Rustyrose words we know what it would cause.
So thinking this, it is very likely that the seal would be weakened.
Obviously Arcadios would be nothing but a puppet in the hands of a Tartaros' member.
About the Dragon Festival, I think it might be some kind of war council of the dragons, in which they all meet, but I guess I am fantasyzing too much now.
Well That's all. What do you think?