I think it all depends on the staff he's playing with. But overally speaking, not a lot of PG in the league average over 18 PPG, i guess it has trended down a notch, in fact I can name only Curry of top of my mind. In fact there are only four other guys.
I would be estatic if gets over 10ppg this year, and adds about 5-6 ppg in his next two years.
Off topic, his defense is right now NBA 1st team, hands down