This is just a theory I came up with while thinking of possible endings. This will contain spoilers from the manga and whatnot so you have been warned. Lol
So the ending a lot of people think is going to happen (and I think this has a high possibility) is that for whatever reason Natsu turns in to his END form and beats Zeref and probably fights Gray. I'm gonna provide a bit of a different theory.
In my theory I say Fairy Tail let's the end of the world or whatever happen, so that they can protect their guild.
Fairy Tail has always put their guild and it's members as their number one priority, as that's where they all found a place to belong. Example: In the Tartarus arc at the end of the Gray vs Silver fight Gray states that even if he is his father he will take him down for the sake of the guild. (Even though he doesn't strike that final blow, if Gray had saw him as a true threat he would have). If forced to choose, he would choose the guild over anything.
Another example: In the tower of heaven arc Natsu pulls Erza out of the tower even though the only way they knew how to stop the thing from exploding and killing everybody was to have Erza sacrifice herself. They just got lucky that Jellal shot it up in the sky for them. Erza even then gets the mentality of "you don't die for your friends, you live for them." Which is probably how everyone in the guild thinks.
In the Tartarus arc when Natsu speaks to Zeref he is told he will have a choice between killing END and letting him live. I believe because of the mentality mentioned above he would choose to "live for his friends."
Natsu is also told that he will arrive to Zeref as either END or Natsu. I say he will arrive as Natsu, but not the Natsu that's trying to defeat him, as his little brother Natsu. One way or another Natsu learns of his origin and decides not to kill Zeref as he is also part of his family. Zeref being touched after being reunited with his brother will decide not to destroy the guild, but can't spare the rest of the world. This will happen because Natsu will try to get closer to Zeref but Zeref will not allow it, since his curse kills those near him, Zeref separates himself from the guild and unleashes the curse on the rest of the world. Fairy Tail then has to live out it's days with no one else left in the world.
Thanks for reading my theory! Now I know this is probably the least likely thing to happen, but it's fun to think about some abstract possibilities! Tell me what you think of it and tell me some of your theories.