I think Rikudou Sennin and Naruto will meet in a subconscious universe and Rikudou Sennin will teach Naruto many jutsu unknown to the shinobi world. Naruto will also treat RS with little respect, calling him "old man" or "gramps" and the RS will bonk him on the head with his staff.
He will teach him superb tailed beast control and also introduce him to his younger son.
And Sasuke will meet the Rikudou Sennin, too. (: