Inspired by an earlier thread on this part of the forum, I believe that every player should have an entirely random potential ability stat, (except that obviously, it should be at least equal to his current ability).
This would mean that it would be less predictable to tell who will be the stars of the future, in the game, (combined with scouts and coaches having far less clue of what a#each player's potential ability level is). SI researchers won;t really know a player's potential ability so this would be entirely realistic. Only a few players in each game should have a really high potential ability level, but there should be a lot that have moderately high level, (but it would be unusual for this to be reached).
Additionally, I think two new hidden attributes should be introduced - "love of the game", and "ability to learn". High stats in these levels would mean, (together with other positive factors already in the game), that career progression and high levels of performance in the short-term are more likely
A player with a high love of the game would be more likely to eat, sleep and drink football regardless of their level of determination or professionalism levels. They would be more likely to stay in the game at an older age, regardless of their level of pay or career progression,. Conversely a player with a low attribute would be more likely to look for opportunities outside the game when things don;t go that well for him, perhaps be more motivated by money, etc.
Certain Mental stats also should be far more volatile for some players than they currently are, perhaps dependant upon how you as a manager, the club's supporters, etc treat the player. I believe determination never changes throughout a player's career? Well, if true I really don;t think that should be the case at all. Furthermore, I believe that determination and teamwork should be hidden stats.