Ok to start off let me just give out a few pieces of evidence before I give my full prediction. ***Spoilers and beware its a bloodly thesis***
In the recent chapters we've seen the Jade dragon Zirconis is in the present time, however he was killed by Achnologia 400ish years in the past [1] and this must mean that the Zirconis must travel back to the past at some point. This leads to my next point, all the way back in chapter 301 [2] Zirconis presumably says to Natsu "Because of you i was..." just as he disappeared, i think that what he said was because of natsu he was killed or something because this version of Zirconis would have recalled all that is currently happening.
Furthermore at the end of 292 [3] its seen that there is a man fighting a dragon, the dragon can be obviously deduced as Achnologia due to the physical similarities and the man is most likely Natsu, due to the very similar scarf that he's wearing.
And finally i found a possible sketch/plan that Hiro Mashima may have drawn for this arc. [4] in this i think the rough translation is "Follow the king's order, Lucy" "A song to yearn, I will never meet Natsu again". So from this two things can be assumed, 1) Natsu has died (ZERO PERCENT CHANCE OF HAPPENING) or 2) that he has disappeared to another place/timeline (most likely option). (This links back to Carla's premonition of Lucy crying and singing before the crumbling castle (lucy singing-->"a song to yearn, i will never meet Natsu again") Also it would make sense as future lucy cried when she was watching Natsu)
So what does this all mean? I believe that after Natsu+Dragon slayers have fought with the dragons and possibly won, Achnologia will still show up and take over the world. So since they can't beat him their only option is to escape, but not to another place but another time; So lucy presumably opens the door and Natsu and the others jump back through time (this would make sense of the old painting of Natsu vs Achnologia), this will mark the beginning of a new arc however Lucy would stay in the present time and this is when Zeref would appear. Eventually Lucy is singing and crying in front of the crumbling castle and passes out and loses some of her memory. She then wakes up opens the Eclipse Door and jumps back a few days to warn Natsu of the 10,000 dragons and sacrifices herself to save past Lucy from Rogue.
So to summarise up in one sentence, Natsu and the others (without Lucy) will travel back through time to kill Zeref/Achnologia marking the starting a new arc THAT WILL BE EPIC!!