I view him as a lock for a few reasons.
He is the most talented scorer/finisher at the pg position that I have seen come out since Kyrie. Not only is he a very good athlete, but the kid can do amazing things with his handle. He can break down the initial defense, then he can choose to pull up deep, pull up from deep mid range, pull up from close mid range, make another dribble move on the drive, and then finish a crazy floater or high off the glass. His offensive skill set is truly that good right now.
He is also capable of being a great 2guard because of his ability off the ball. He seems to always be in the right position on the offensive end of the court. People are enamored with Ball's passing, and while not as flashy, Fultz can make the vast majority of the passes Ball can. He is a gifted passer at this stage. If he was on a team with better players (like UCLA) his scoring would obviously be lower, yet his passing would be able to be on full display. I watched one of his games earlier this year, and his teammates missed 3 bunnies that he created with his next level passing.
He also has a ton of "defensive potential." At this stage he isn't great at it yet, but he makes plays all the time that have you going wow. Since he is such a high level athlete, every couple games he will get an insane block, or my favorite, pin a shot on the backboard as a pg.
Basically, he looks like a complete guard. Incredibly versatile, can dominate on or off the ball, and his only current weakness, he has all of the physical tools and drive to get better at.