Why cant there be a 3rd part?
There
might be a 3rd part to the story.However,it isn't clear if it'll be in the manga since people keep saying the end of the 4th shinobi world war arc is marking the end of the manga.If there was a 3rd part,I'd bet the prologue of it would be the movie coming out next december in Japan,with the New Era Project rumor being the actual 3rd part.I hope more politics and adventure will be featured since all the story up to this point has been about the developpement of Sasuke,Naruto and the shinobi history through time since the S06P.
P.S.

on't expect that hypothetical 3rd part to take too much time as Naruto has an actual story to tell and isn't a serie like pokemon where a story plot is almost non-existent,and I don't think it's supposed to be compared to The NeverEnding Story.