Of course he's going to improve. But whatever level he is at the moment is a good indication of where he will end up. The likes of Pogba, Scholes, Xavi, Alonso, etc didn't just suddenly develop the ability to make amazing passes when they were 18. They always had the ability, and as the matured they then improved on that both through technique and decision making. The same will be true of Bellingham. If he's only average at the moment then it's highly doubtful that it's ever going to be an actual strength of his game. But if it's something that he's already good at then it is a promising sign that he will have that along with his obvious ability with the ball at his feet. Hence my question to anyone who has actually seen him play, since that aspect of his game is pretty much ignored in the videos.
The defensive attributes are more likely to change significantly as he matures, but it's still interesting to know where he is at the moment.