I think with the popularity of the Persona series sky high right now, I think we will see Persona 6 much sooner this time, within 2-3 years around the end of the PS4's life cycle. This may sound ridiculous but Persona 4 was released 2 years after Persona 3 in Japan, so I think Atlus is working on Persona 6 right now.
There'll probably be a couple of spinoffs in the mean time, most likely Persona 5 Arena or something of the like, a fighting game. And maybe something like Persona Q starring the P5 cast.