Here me out on this one guys! Polyusica said that Natsu's tumor wasn't ethernano, rather it was something far worse.... Brandish enlarged the tumor last chapter and given the circumstance that both him and Lucy are being held captive by Dimeria what if we see E.N.D. awaken in the next chapter? The next chapter (503) is called "The Last Sight I Beheld"... since Fairy Tail is told from Lucy's point of view what if she witnesses Natsu awakening as E.N.D.? And that's how the chapter ends, with his awakening? It's just speculation but with the pace of the manga it would make sense to see E.N.D. Right now and since Gray knows the secret now we would see how that plays out... But it would be added pressure for Fairy Tail to win against Zeref and the Alvarez Empire.... Let me know what you guys think!
Link to chapter: http://m.mangahere.co/manga/fairy_tail/v38/c502/