I tracked Fox since Calgary drafted him. He does have that 'something special' in him. To those that say his ceiling is only a PPQB are flat-out wrong. Want to see how well he has been developing? Watch his first WJC and then watch his 2nd. Night and day with how good he was defensively. Huge strides.
He is slightly undersized and needed to put on some weight and work on his skating a bit more, but he was becoming a much better player, hence increased talk about him since the draft. I bet he becomes a top 4 NHL defencemen along with being the team's PPQB. I am not sure if he will become 'top pairing' good - he will need to become a bit faster and stronger IMO to handle the tougher assignments - but I think it is a safe bet to proclaim him as a future top 4 defencemen who will probably lead your defence in points without being terrible in his own end either.