Random Speculation about Zeref, Ancologia, and Layla Heartphilia (& indirectly Lucy)
First what we know:
Ok, so we all know Zeref supposedly made Ancologia into a dragon from the dragon slayer he was before. However how did he do it? It was also implied Ancologia is Zeref's tool, to at least some degree, though how much free will he has is unknown. Ancologia can also detect Zeref from a vast distance, since when Zeref finally lost his good heart, Ancologia was summoned forth.
We also know Layla Heartphilia knows about the one magic Zeref possess, yet no one else seems to know about it at all. Which is odd. Layla was also sick most her life and "died" in the year 777 the year all the dragons disappeared.
We also know the Spirit King for some reason thought Lucy was special enough to visit in person when no normal spirit mage could ever do such a thing. Finally Loki and all Lucy's spirits can detect and communicate to some degree with Lucy without even being summoned.
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Now my speculation:
So I'm wondering if Layla Heartphilia was the ingredient used to make Zeref into a dragon. Simply put, Layla was once Draco the Celestial Spirit Dragon. However what if Zeref got her key and stole her dragon hood status and gave it to Ancologia, which turned Layla into a mostly mortal human... that just might be the thing that both made Layla Heartphilia sick due to being absent from spirit world and disappeared/died in the same year as the dragons. Plus it explains why spirit king would care so much about Lucy's feelings, since if true in a sense Lucy would be one of his subjects and familiy. It would also explain how Ancologia could detect Zeref from where ever he was prior and possibly why Ancologia was flying all over the world (he was seeking Zeref).
Anyways just random speculation. I still don't have an guess as to what significance 7/7/x777 has or the dragon king festival... just a theory of the relationship of Zeref, Ancologia, Layla, and Lucy.
Tell me what you think about it.
EDIT: Oh as to the 400 year age gap... I suspect that would tie into Natsu and Gajeel being over 80 or stone statues
Plus Zeref is over 400 years old and very young, so this really isn't a detriment to the theory. And Hades didn't age for the last 48 years of his life either...
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Random speculation about Arcadios personality swap over Zeref and what he's fighting for:
Maybe Zeref had Arcadios under some sort of spell to become his mindless slave (like Jellal to Ultear but on a more epic level since it's Zeref) and that's why he was acting all lets worship Zeref. However later Princess Hisui who also has powers like Zeref (if she's the robed girl, which I suspect she is and will be revealed next chapter based on the chapter title, since I believe last Chapter Hisui snuk away from the guards and I suspect it was to sneak away to meet up with Natsu and crew) noticed it and as a result freed him, that's why he's against Zeref and pro Princess now but wasn't then.
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Oh least I forget... Hiro's twitter confirmed even though he doesn't really plan much for FT he does have at least 3 more arcs minimum actually that he wants to do (each arc pushing 1-2 years of late would mean 3-6 more years worth on FT,but based on the ever increasing amount of characters I'd wager 6 on the high end). However he did previously mention arcs like Fighting Festival just randomly came about on a whim. So we could get a lot more than 3 more arcs too.