Even though he was still an absolute champion, when I think of players who could have been better, Allen Iverson comes to mind.
He's last 2 or 3 seasons of his first stint at the sixers were damaged because of his ego and putting himself ahead of the team. Denver is where I believed he could have played his best but didn't quite live up to his potential. Pistons he had similar problems that he had early in his career with Sixers. Grizzlies is not even worth talking about and back to Sixers his age had caught up to him.
I honestly thought he could have been in the top 3 point guards of all time. Maybe an attitude adjustment and putting the team first he could have been. He is still one of the greatest point guards anyway IMO and one of my all time favourite players, but he could have been more.
He could make up for it by coming to play in Australia in the near future