Fultz has a complete offensive arsenal. He can score at any distance, in close, midrange, deep. His body control is elite. With his reach and high release, it is extremely difficult to even bother his shot. Even in heavy traffic, Fultz plays calm and under control, shoots over anybody with that 6'10 reach and ability to create space.
So I'm going to say he's easily going to be a 20+ ppg scorer in the NBA. It might not happen until year 2 or 3 but it's going to happen. 15-18-21 is not hard to picture as his scoring progression from year to year.