this thread has been made way too many times. but final villain as in what? like the final battle against good and evil in the war? madara will probably be the final villain as far as the war goes.
but if you're referring to who will be the final overall villain id say orochimaru.
definitely not sasuke
i don't understand why people don't see the point of naruto. the theme is peace.
the three main points that will be achieved by the end of the series is, naruto will be either hokage or have just surpassed all of the previous hokage (which if the hokage come into the war that fact can be proven that naruto surpasses all previous hokage), peace will be attained where people truly understand each other, and LASTLY: Naruto will have finally brought Sasuke back home to the leaf village.
That was the initial first point of shippuden his goal of being hokage wasn't mentioned all that much, he was more focused on getting stronger to bring sasuke back to the village. as far as the peace thing goes that was introduced in this series, and naruto believes in that peace and that is why naruto will not kill sasuke. Sasuke will most likely be around after the war unless obito the ten tails or madara kills him or he sacrifices himself.