User blog comment:IamJakuhoRaikoben/Chapter 418: Challenger, Review./@comment-24546844-20150202172212/@comment-7418318-20150202173022

Hmm... now that you mention Natsu and boss... maybe he will be one of those hostile bosses. The "message of flame" might be his awakening as E.N.D. and the arc will be about Lucy gathering the old gang so they can turn Natsu back to his old self.

It does seem a bit too soon for such a development, but maybe this "message of flame" is just a first step and will begin his slow transformation.