I know Grant is strong and moves his feet well. He also has a solid understanding of how to play defense and what his role is on offense; but, is there any way for him to work on his sheer athleticism?
Sure, of course. He needs to lose some of that fat, for a start. He's got a lot of capacity for improvement with his hip joint muscles, too - he just needs to put in the time.
We can be confident about his work ethic; probably he'll show up to training camp 2020 looking (and especially moving) noticeably differently.
It's not likely that he'll ever be known for his first step or his shot-blocking, but there's more than one way to get to the rim or to protect the rim on the other end. Then, too, as much as athleticism, anticipation is a key element in playing good team ball, and that's already one of Grant's strengths; again, in Year 2 he'll likely take a step up in that area, after a season plus playoffs of rotation minutes.