It's really not that much a matter of physical or athletic dominance. At 6-5, Kostov has just decent size and average athleticism for a wing. What makes him a unique player is his unbelievable array of skills to put the ball in the basket. Shooting, slashing, even posting up his defender, he plays all over the court. He can release his jumper from any place, in any sort of fashion. He finishes around the hoop with both hands and a great variety of layups.
You name it, he has it. Truly an amazing display.
Kostov doesn't care if he has a defender in his face; he will fake, change directions or just unexpectedly go up for the jumper. There seems to be a special connection between him and the net, to the point that most of his perimeter shots gently swish through the basket.
Still, Kostov has slowed down in the last two games. Turkey and Spain delivered special defensive attention on him, and while he did a good job passing the ball off the dribble against Turkey, he stubbornly and unsuccessfully looked for points versus the very strong Spanish defense.
Kostov's potential seems limited by his physical shortcomings and maturity, so talking about a hypothetical NBA future is not reasonable at this point. However, he should become a very nice player for Europe.