Sasuke's going to get rinnegan. We're going to learn that mikoto uchiha was infact a senju who defected. We'll learn that senju + Uchiha = Rinnegan = The requirements for the elder son. Therefore, sasuke is going to be a pseudo elder son.
Then, we'll learn that uzumaki is actually a direct decendant of the younger son, and naruto is a pseudo younger son.
Then....
Naruto and sasuke will make a kid who'll be the sage. True story, i read kishi's script.