I could give you my answer, but my answer is nothing but a guess. All any team can do is make their best guess based on as much information that they can find. Which of course is more than I'll have. Or you'll have. If a player works hard, he can improve his skill level. So if Doncic is a hard worker, he'll get better. The one thing a player can't significantly improve is his athleticism. If you don't have great lateral quickness, you can improve is a little, but in most cases, not enough to make a big difference.
So my guess would be, that if you have a player like Bagley for instance, who is a freak athlete, but is a little weak in the skills dept, but he still puts up big numbers despite that, then you probably have a player, who with hard work, can be significantly better down the road. Translated that means, if he's good now, without those skills, he'll likely be a monster with them. = Very high ceiling....
On the other hand, when you have a player, like Doncic for example, who is maybe, a little better than average athlete, but highly skilled, who put up above average numbers in Europe, is likely to get a little better with hard work, but the jump will not be as significant as the jump Bagley made. Lets be honest, freak athlete's that are highly skilled are rare, and the chances of any of these top players becoming that are slim. Because if they become that, then you have the next Michael Jordan, or Lebron James. Those are once in a lifetime players.
But the chances of a Bagley becoming a Paul George, Dwayne Wade or a Kevin Garnett (referring to talent level only) isn't that far fetched. But I think the chances of Doncic reaching that level is unlikely. Doesn't mean he can't make an impact or be an all star, but I can't see him ever approaching superstar status. Of course I could be dead wrong! But for the most part, I think history is on my side. If you lack that elite athleticism, it's more difficult to reach that status, but not impossible. However, I'll take an average athlete that's highly skilled over an unskilled elite athlete all day long. Assuming I'm talking about a finished product.