Kishi already said that he will stop with writing Naruto after part 2. Well, maybe he changes his mind but I think he wants to write about something else, like he also mentioned.
But I really can't see a manga, such succesfull and worldwide known, to end just like that. I am quite sure some other writer will buy the rights and will write a part 3. I am 95% sure about that. The only thing I hope then, is that it will be a good writer so that it will remain good and interesting to follow.
For me, a part 3 may come, FOR SURE!