In my blog, Natsu vs. Acnologia, I listed the possible ways Natsu's fight with Acnologia would go down. My top two favourite are Natsu in Dragon Force against Acnologia in Dragon Form, and Natsu in his normal state against Acnologia in his human form. Seeing how the latest chapter ended, I have a thought of what will happen in the next one.
Natsu breaks into Zeref's location and challenges him. Zeref tells his subjects to stand aside, insisting he'll deal with Natsu personally. Before they can begin fighting, Zeref says they must wait for their third opponent. Natsu, at first, is confused until he hears something he hasn't heard since the Tartaros war. Images of Igneel dying overwhelm Natsu's head. Acnologia arrives, shocking everyone and, out of anger, Natsu enters Dragon Force and turns his attention to avenging his foster father, attacking Acnologia head on. Zeref, left disappointed, tells everyone to proceed with the assault. Just then Jellal and the Crime Sorcière arrive. Shortly afterwards, Cobra/Erik hears Zeref's thoughts and with eyes, or rather eye, widened, stares up in the sky at an enraged Dragon Force Natsu attacking Acnologia.