In my previous blogs, I stated my desire to see Natsu fight Acnologia in his human form. In the chapter 400 Years, the dragons brought up Natsu, Gajeel, Wendy, Sting and Rogue to take down Acnologia. Natsu left after the Tartaros war to train to kill Acnologia. He returned in the Avatar arc to prevent Rogue from going down the dark path his future self went down. Not sure how much strong he got before Avatar or if he ever won against Gildarts. Is the Fire Dragon King Mode the best he got? Can he enter Dragon Force at will like Wendy?
So, Natsu isn't quite there yet. I realize that if Natsu went head to toe with human Acnologia, let alone his dragon form, he'd be beaten pretty hard. Here's a better thought. One way Acnologia would revert back to human is if Fairy Tail unleashed Fairy Heart and instead of kill him, return him to human. Perhaps Natsu could attempt to fight Acnologia on his own and when Acnologia is about to kill him, Gajeel, Wendy, Sting and Rogue could save him, and maybe even Laxus and Erik as well. The 7 Slayers re-team again to take on the Dragon King.