I can see him improving in aspects of the game that aren't completely tangible. Defense, decision making, playmaking, leadership, toughness, and so on. Athletically, no doubt he's in his prime, but I think experience is an underrated or overlooked quality on this board. You look at the past champions, and it's usually guys who've been in the league awhile.
I remember when people used to say the same thing about Lebron.
Durant hasn't even sniffed his prime yet dude.
This is his prime. The guy is and will always be a one-dimensional scorer. He isn't just all of sudden going to lead the fastbreak or anchor a defense. LeBron was dominating before he even had a jumper/post game
He's shown improvement in his defense every year he's come back in the league and I think he can continue to improve on his offense.
But the thing that can bother me is that he might depend on the refs WAY too much, that he might won't make these certain adjustments to his game when his jumpshot isn't dropping which will make him rely on his length and athletic ability more and more, until he gets older and doesn't have that much of a refined game.