I'm not sure. At this point in the story, Naruto is far too powerful to really be challanged by anyone other than Madara and Sasuke. A part III wouldn't have enough villains that could really challange him. Naruto would have to go MIA for most of part III and it would have to circle around the rest of K11 for there to be any real story or struggle otherwise it would be over really fast.
I mean Kishi could just make a ton of new extremely OP Villains, but their is little to no room for further Growth for naruto's power other than him becoming an adult and Hokage.
Still, at least there would be more to read and watch. So that would be a plus. But in all honesty I am really just ready to see the series end. 15 years is long enough XD