User blog comment:IamJakuhoRaikoben/Chapter 373: Life or Death?, Review./@comment-2214433-20140302221321/@comment-5593752-20140303104402

I have a theory about END, that might be a little out there, but I think END might be Silver's Demon form; and if the theory that Silver is in fact a future version of Gray that would certainly be a huge choice for Natsu to make.

It could be something like this at some point in the future Gray, for whatever reason, becomes a demon (END) then somehow travels back in time and sets up Tartarus. Now as the battle with Tartarus goes on Silver eventually reveals himself as END but is too powerful for Fairy Tail to beat and he is on the verge of killing everyone; Natsu however realizes that if Gray was to die in the present he could never become END. Forcing him to choose between dying himself or killing his friend.

I know its out there as a theory, has holes in it, and admittedly the timelines don't really line up, but I think it would be a pretty interesting turn of events that I wouldn't mind seeing.

Doubt it'll happen though. Also if someone else has already suggested a similar theory I apologize, I read a lot of comments but not all of them.