I agree with your point about Natsu being initially unable to defeat zeref. I think that he might have to experience significant loss in order for that to come out. though I don't think it's gonna be lucy- they kind of already did that and it would be lazy to do it again. I'm think Erza or Gray, or Happy. I think Gray will find out that Natsu is END and will try to fight him but after a brief fight will ultimately realize that it serves neither of them any purpose to fight, with zeref being the major issue. I think Natsu will end up eating acnologia's flames and enhancing his dark END powers to defeat zeref and mavis will come around to cleanse the darkness out of him, but maybe wind up having to seal his magic away or himself in his entirety.