I see what you mean but if Grey is able to best END natsu surely grey would then be more suited in taking down zeref. So I think it would make more sense that grey will have a part in stopping natsu but not just solely him, I also think Lucy will probably be mixed up in this somehow.
And Erza, Happy, Wendy, and the rest of Fairy Tail.
If Natsu goes out of control, I suspect it will be a story arc where all the characters can return all the favors they owe Natsu for all the time he saved them.
But I also think that when Gray first learns that Natsu is END, there will be some tension between the two of them or a while.