And Erza, Happy, Wendy, and the rest of Fairy Tail.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.
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.