Here's an interesting thought: Hidden class system.

Have a number of (hidden) character classes. The game determines which class a character best falls under (based on that character's actions), and grows their stats accordingly.

For example: You have a character use bows and white magic often. The game notices these tendencies, and classifies that character as a 'white archer' and assigns that character to the 'white archer' growth rates. You have a character that uses double-axes and no magic. The game notices these tendencies, and classifies the character as a 'berserker' and assigns that character to the 'berserker' growth rate. You have a character that uses sword, shield, and white magic. The game notices these tendencies, and classifies that character as a 'paladin', so on and so forth....

The challenge would be making a system detailed enough to account for many styles of gameplay, and also able to change the types of growth rate in conjunction with a change of play style.

Thoughts?