I think Hanifin is going to be really good, possibly one of the best d-men in the league, but there is that chance with him that he doesn't turn out to be that guy and is just a solid guy in your line up. I can't help, but think of another just as highly touted US d-man who had the same kind of hype, but to me is just a solid one not a franchise changing one: Eric Johnson.
With Marner I think you are getting as close to a can't miss prospect in the draft (besides McDavid and Eichel) as you can. His creativity, hockey IQ , mobility and 2-way don't allow for him to be average. His brother didn't finish growing until 19 and is 6"3. So he could still end up 6"0 or taller.
Despite the rarity of elite D, we can't afford to not nail this pick. Also, there are a couple of pretty good elite D in the draft next year if we are in the 3rd to 5th pick range named Chychrun and Day that would nice in a Leaf uniform.