Some notes:
1) There are boom players (those who exceed their potential) and bust players (those whose potential fall rapidly after they are drafted).
2) Coaches will help a player reach their real potential faster, but do not effect the boom or bust cycle.
3) There are certain attributes - strength, speed, acceleration, intelligence, passing arm, and passing release (I think) - that do not have a potential to increase that is shown. I'm almost certain that none of the attributes will improve. Although I'm certain that they do degrade over time.
