Hart is a mid level goalie at best. All stats point to this . Can he improve? Yes. Will he ? Probably not. He is firmly a mid level # 1 . Probably around 12 to 16
I think anyone who has spent a significant amount of time watching Hart play goalie (and the Flyers not play defense) knows pretty well who he is. Goalies are mercurial, but on a half-decent team his floor is kinda "above average starter" and his ceiling is "will be top 3 in the league for a few years at some point in his career." He's fundamentally rock solid. Not crazy athletic, just reads plays unbelievably well. Real-deal NHL starter at a young age, and one who has had to truly earn most of his wins in the league to date.
He honestly doesn't have to get much "better." He just needs to be playing on a real NHL team with real NHL players, coaches, and training staff.
Goalies don't have much trade value, though. Really good ones who are his age? Who knows.