From what fans speculate, Natsu would either become E.N.D. or a Dragon to defeat Acnologia. The last one seems very unlikely, seeing that Igneel prevented Natsu's Dragonification. If you ask me, the Dragon Slayers more likely to become Dragons are Laxus and Cobra. They're the only Dragon Slayers who didn't have Dragons hiding inside them creating antibodies and it wouldn't make sense if Second Generation Dragon Slayers weren't at risk of Dragonification. Here's a decent scinario in the current story arc for Laxus to become a Dragon:
A three way fight between Natsu, Zeref and Acnologia (in human form) begins. Zeref tells Natsu the truth and Natsu stops fighting and and becomes frozen, almost as if he lost the will to live. Acnologia, upon hearing that Natsu is E.N.D., attacks him with brutal force and Natsu lets him. The Fairy Tail members see this and with that, Levy, Alzack, Bisca, Reedus, Cana, Macao and Erza send their fire to Sagittarius, who shoots flaming arrows to Natsu, who, to their surprise, doesn't eat it, making them realize he's not even fighting back. Acnologia then turns back to a Dragon and attempts to bite off Natsu's head but Laxus, in a flash of lightning, rushes to Natsu's aid and attempts to fight Acnologia. Eventually, he over uses his Dragon Slayer Magic and transforms into a Dragon. The two dragons would go head to head, probably with the inexperienced Laxus being over powered the same way Igneel was. Seeing this would give Natsu the encouragement to fight and he does so, running to Laxus's aid, punching Acnologia in the jaw, knocking out several of his teeth, to everyone's shock.