People have said that the only way Natsu could defeat Acnologia is by becoming E.N.D. or becoming a Dragon (the scar on Natsu's face that Zeref left him kinda hits that possibility). I believe the Salamander is strong enough to hold his own against the Dragon King, at least with Dragon Force and Fire Dragon King Mode. He just lacks the Magic power to match him, now that Igneel's Magic power is gone. Natsu can still use Fire Dragon King Mode, only he uses his own Magic power as a substitute for Igneel's. I don't think Acno will kill Natsu right away. He may torture him to get back at Igneel for taking his arm. That said, Chapters 475 & 476 introduced the Third Origin, which adds a new possibility on defeating Acnologia. Now, besides becoming E.N.D. or a Dragon, I've narrowed it down to four possible sources of power that can completely substitute Igneel's:
- Eating Fairy Heart - We all how reckless Natsu is. At the Tower of Heaven, he ate Ethernano-contaminated Lacrima to try replenish his strength. It almost killed him but he entered Dragon Force. On Tenrou Island, he ate Zancrow's Divine Flames to defeat him. This sounds like something Natsu would do. If Cana doesn't destroy Fairy Heart and Zeref or any of the 12 don't get to Fairy Heart, I believe Natsu would eat a piece of Fairy Heart and use it as a substitute for Igneel's power in Fire Dragon King Mode. Infinite Magic Power + Fire Dragon King Mode. A killer combo. That should enough to kill, if not wound, Acno.
- Again, Third Origin Release - As shown in his fight Zeref, Natsu was, and is, willing to give anything to save his friends. Which includes, as you know, his life. His Magic is no different. Is Ultear the only one that can release Second and Third Origin? She could've taught Jellal those spells during the 7 year time skip. If they found some way to unlock the Third Origin, it would extremely useful. Third Origin Release + Fire Dragon King Mode. That could work.
- Taking Over E.N.D.'s soul - Am I the first one to think of this? Remember when Laxus transferred his Magic Power to Natsu, who then gained Lightning Flame Dragon Mode. The only humans, besides Zeref and Acnologia, that could possibly go toe-to-toe, if not hold their own, against E.N.D. are Gray, an Ice Devil Slayer, and Mirajane, a Satan Soul Take Over Mage. Mira can Take Over demons and their abilities. First, she'd try fighting E.N.D. using her Satan Soul forms, including Satan Soul: Mirajane Seilah (maybe even command herself to release her limit) but E.N.D. easily overtakes her. Then, she could Take Over E.N.D.'s Flame Curse (we don't yet know what the exact name of E.N.D.'s Curse is, but I heard E.N.D. is a Flame Etherious, so for now let's call it Flame Curse), like she did with Seilah's Macro, and tries to fight the Flame Etherious with his own Curse but gets her ass handed to her. When Natsu, inspired by his friends' unwillingness to give up on him, tries to break free from E.N.D.'s possession, Mira decides to transfer her Magic Power to Natsu, who knows a little Transformation Magic. Natsu would then Take Over E.N.D.'s soul and merge it with his own, or at least Take Over E.N.D.'s abilities (Flame Curse and Etherious Form) while the rest of E.N.D.'s soul is destroyed by Gray. Natsu, as you all ready guessed, then uses Flame Curse as power for Fire Dragon King Mode. Or even combined the Flame Curse with his Fire Dragon Slayer Magic to form some kinda Etherious Inferno Dragon Mode (Natsu thinking Inferno starts with the letter "N" for comedic effect).
What do you guys think? Sound like something that would happen in the manga?