i'm expecting Naruto's training to master sage mode will be the factor that will have him complete his rasengan. yamato said when naruto arrived against kazuku that it was only half way done. chouji even asks since the technique is a "shuriken" why cant he just throw it. in a few of the panels we saw during his training we saw massive trees cut down and huge scars in the ground, implying something very destructive. one can only assume that he has the technique completed or damn close to it.