That is a perfectly fair question re. improvement. His box score statistics are actually down on average since the WJC compared to before then, so you can't really point to that. Although I will say that part of the reason why is simply because he hasn't been on the same line with Leonard since the break, which hurts a player who primarily racks up points through assists.
I think it's worth saying that part of the reason why Hagens might not be making leaps and bounds is because he came in as a very polished player. There really has never been much to dislike about his game. That's probably why he was the presumptive top prospect going into this season, and none of that has changed. He is decisive, responsible, willing, technical, and skilled. And he pretty much always has been.
The comparison to Will Smith is a good one because there are obvious parallels to the two given that they have followed almost the exact same trajectory. And though you're right that Smith did get better as his college season went along, that is because Smith had much more to improve upon than Hagens did. Let me be clear that I love Smith's game, and I think he has rare creativity that surpasses that of Hagens. But as I'm sure you know as a Sharks fan (I think?), he is prone to being overly ambitious, which results in mistakes. Plus he can get caught being floaty more often than you would like for a player you dream of being a 1C (in the abstract; not in the Sharks' case because of Celebrini). Hagens has not had those flaws to improve upon. He creates boring but high probability chances more than dazzling but risky ones, he is responsible with the puck and very rarely turns it over, and he is engaged in both zones. Those are all things that Smith needed to (and still needs to) get better at that Hagens didn't.
The other thing I will say about Hagens is that I think he is unfairly criticized for not being the driver on his line. He walked into a situation with Leonard/Perreault where he is the new guy alongside two of the best players in the world who aren't playing in the NHL. And those two guys have been playing with each other for years. If Hagens had tried to commandeer the line to fit his will, it would have been a red flag to me. I think it's a sign of maturity that he was willing to adapt to his circumstances. And I think adaptability will be a hallmark of Hagens' game for the long haul. Since he does everything well and there is no diva to him, he can fit in with pretty much any linemates. Again to draw the comparison to Smith, he is the type of player who needs certain linemates to maximize his talents; I don't think that's the case as much with Hagens.
Maybe this is a bit of an apologist's take, and I would probably argue this is why almost every prospect, including Hagens, would do well to come back for a second year of school. Hagens' biggest improvement will probably come from spending more time in the weight room, which a college schedule allows for. He would also be in the Leonard/Perreault role next year if he came back rather than being the guy who has to take a backseat as a freshman, and that allows prospects to assert their own will a bit more before returning to the pipsqueak role in the NHL.
If I had to name any one area in specific that Hagens has improved, I would say it's his compete/physicality. He punches above his weight class in the sense that you very rarely see him get bodied, and he is more than happy to get in the mix both to separate the other guy from the puck or else help his teammates on the boards. And that is something that I have noticed tick up as the year has gone on. Even in games in the second half where he has not put up any points on the scoresheet, I notice him getting involved consistently in a way that I didn't in the first half. And I think this underscores the floor with Hagens you're mentioning and that I agree with. He's just a very good player that any NHL team would be happy to have.