I think some things can be improved on. Especially as you mention, stamina. There are some rules of nature that apply. Such as, you can either run fast, or you can't. You can either jump high, or you can't. You can either throw a baseball above 90 mph, or you can't. And in general, your pretty much stuck with whatever god given talent you have. However, all those things can be improved on to some small degree. I can almost guarantee you that you'll jump higher wieghing 265 pounds than you will wieghing 280 pounds. I suspect that you'll be significantly quicker as well, and possiblly run a little faster.
In short, I think Cousins can improve on, or lets say polish the athleticism that he has, and he can certainly improve his stamina. I know some of the criticism about Cousins is that he plays below the rim. Not a totally accurate statement since he can easily dunk the ball. But I know how it was intended. So here's the good news. Players that play below the rim are usually less prone to the major injuries that sometimes plague the high flyers. What goes up, has to come down, and sometimes the landings aren't exactly as intended.
