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I borrowed this idea below from someone who released his own expansion for D&D (BX version, I think), and modified it to fit my campaign, although I'm not sure if I will pull the trigger on it. This may be of interest to some of you.
Improving Ability Scoresby Craig Antoun
Characters may attempt to increase their ability scores once every four experience levels up to 20th level (i.e. 4th, 8th, 12th, 16th, and 20th). For demi-humans, treat Attack Ranks as levels for this purpose. To attempt to increase an ability score, the character selects one score, then rolls 1d20 and consults the table below. If the roll is equal to or higher than the number in the Roll to Improve column for the Current Score, then the selected ability score increases by one point. This attempt can only be made once per designated level, and there is no penalty if the character should fail the roll. Ability scores cannot be increased beyond 18.
Current Score Roll to Improve (d20) 3 10+ 4-5 12+ 6-8 14+ 9-12 16+ 13-15 18+ 16-17 19+ 18 n/a