I think their will be a third part. Even if they stop the Edo Tensi, Tobi(Madara) still knows how to use the technique now. Zetsu can still make another clone of Madara and anyone else he wants. If the Edo Stops Madara's body is not sealed and still out there to be resurected again. Sasuke will still seek vengance and he and Naruto will not meet up yet. You still have Sugeitsu out there who is evil and comes from a powerful clan. Naruto as powerful as he currently is still hasn't reached full growth and maturity. Also all there is, is the five great nations joining together no says some other smaller nations doesn't have someone powerful in them, Pain was from Village hidden in the Rain, just as Hanzo and Oro started the Sound Village. So he can still go on