I don't think he will improve much more than what he has now. Probably one more boost from the other tailed beasts chakra, but that will probably just be temporary.
He will certainly learn new jutsu within 20 years, but doubtfully become stronger, he will just become more versatile so he can beat more kinds of opponents and withstand genjutsu etc..
Also he will have trained his own team, he will learn how to be a leader, how to lead a village, and then he will be hokage.
If he gets some new permanent powerup it will probably be the rinnengan, but that would just be boring, we've seen enough people with rinnengan, and it seems more likely that sasuke awakes it.. But with the right jutsu at your disposal and the current strength that naruto has, even that will be no challenge U_U