User blog comment:CNBA3/FINAL END/NATSU/ZEREF THEORY!!!/@comment-1.39.33.152-20150911121740

Gray will eventually fight with natsu as E.N.D is a flame demon (silver's last word) and natsu being a fire dragon slayer makes sense. Zeref's full name is Zeref Dragneel, it may be that Natsu is the younger brother of Zeref and Zeref turn him into a demon but the power is not yet unlock. No one opened the book of E.N.D yet so it may be that with the opening of book Natsu will awaken his demon power ( Igneel also warns Natsu not to open the book). With Silvers last words and the ice devil slayer the only last momento to Gray, Gray will despise E.N.D and may have conflicts with Natsu. (Just a maybe)