Good day everyone!
first of all ignore my broken english, i didn't had any english lessons, i learned it myself with playing video games and Mangas/Animes ofc :P
We all know E.N.D is a Flame Demon, but what kind of magic he use?
Silver and I think Igneel mentioned that E.N.D. is a flame demon/flame magic user
http://www.mangareader.net/fairy-tail/394/20
Before Igneel teached him Dragon Slayer Magic, probably Zeref teached/gave Natsu Flame Magic.
It has to be very powerful Magic, since E.N.D is created to kill the strongest Black Wizard, even Igneel knew that E.N.D is strong.
Black Wizard Zeref and
Flame Magic,
What if Zeref the Black Wizard, is creator of God Slayer magic?
Zancrow learned God Slayer Magic from Hades, Hades was wandering on Zeref's paths, he probably found a book from Zeref wheres written how he teached E.N.D this Magic.
So Hades trained Zancrow this magic but Zancrow his Magic skills was very incomplete. by the way alot Peoples from Grimoire Heart usig very similar magic to Zerefs Demons ( Tartaros Guild ).
When Shelia used Sky God Slayer Magic, Jellal said: "That girl couldnt possibly have a connection to Zeref, could she?"
http://www.mangareader.net/fairy-tail/288/19
Yes she have Jellal and the connection between Her and Zeref, is God Slayer Magic
Back to Zancrow, now you're asking yourself why Natsu couldn't ate Zancrow his flames? Don't come with the Oil theory this is just some random BS. yes zancrow said, he cant eat the flames cos a God is above a Dragon, this is just some BS again, Zancrow is ver arrogance thats why he said this. Natus couldnt ate them because Igneel was inside Natsu and blocked the Magic, probably Igneel was afraid that the seal could break since this is Natsu's old Magic, Natsu empty his Flame Magic in order to eat the God Slayer Flames, so Igneels Power was weak/gone to block it.
Wendy didn't try to eat Shelia's Wind Magic, even she really wanted to win, if she really wanted to win why she didnt do it?
So the Oil theory like i said is just random BS. Yes Wendy saw how Natsu couldn't eat it, but she saw Natsu absorbing/eat Laxus his magic. So she really could try it and win the fight, if Mashima showed how Wendy couldn't eat the flames i agree on the oil theory, but i think Mashima didn't show how Wendy try to eat the Magic on porpose.
Alot peoples want to see E.N.D and Natsu again with God Slayer Magic, why not doing both stuff on same time?
Natsu's God Slayer Magic would be complete, he even could have "God Force", a power enough to kill "semi-gods" like zeref and Acnologia ( Why Acno fears E.N.D that hard? because E.N.D have the Power to kill a God like him and Dragon Slayer Magic ). We all know there will be more Arc's after this, the most of us thinks Anksheram is the last enemy, a real God. So it wouldnt be bad that Natsu can use it.
I could say more but i forgot it and need to go lol
Iv'e alot more theories will write them if i have time and if you want.
If you find any misspellings or grammar failures, forward it to me, i will change it soon as possible.
Greetings Into the NighT