Hmm, well one happy way to end all this is to stick Zeref in a Lacrima with Mavis so they can be together again while not killing anyone. Natsu doesn't die, Zeref isn't killing everything so he's happy, Mavis gets her old boyfriend back, the main threat to Fiore is gone, and all they have to worry about is a murderous apocalypse dragon, a bunch of overpowered foreigners, and a shitton of unanswered backstory questions. And hey, the dragon and the foreigners might just take each other out.