User blog comment:IamJakuhoRaikoben/Chapter 455: Magnolia's Defensive Battle, Review./@comment-201.156.192.145-20151005060350/@comment-9595671-20151005112439

I'd think this is more likely to happen when Gray and Natsu are in the same place. That way Gray ends up realizing that killing END means he has to kill one of his best friends.

As of right now, I don't think the two are gonna get anywhere close by anytime soon. Plus it doesn't look like Zeref's anywhere nearby right now.