Via an interview posted on Anime Matome, Kishimoto revealed some of his plans for his next series.
He's already started working on the draft for his next series and has the main characters designed, though he hasn't showed his editor yet. He had been thinking about what he wanted to do next while working on Naruto, and had already decided the genre would be science fiction.
He isn't interested in doing another weekly series, but when asked if he was going to target a monthly magazine, he instead said he was interested in making the series for the digital Shonen Jump Plus platform, as it has less strict page count and deadline restrictions.
He's not prepared to announce it yet, but is challenging himself to surpass Naruto with it, and hopes it will be an even bigger hit.