I think it's Ankhseram. They might have another motive for cursing Zeref for reviving Natsu as E.N.D or do they plan to destroy to humanity like Acnologia?
P.S: Before you reply think about why is Ankhseram called The God of Life and Death for a reason? Why does Ankhseram have two things...
Hiro might make an arc based around Acnologia if he leaves during the war. I mean if THERE WILL be a possible arc based around him it might generate into his backstory more. Just Saying.