I propose to do without ability scores and just use ability modifiers (see rule page). This is intended as a simplification of the game and the character sheet. It is based on the observation that in the official rules there are few uses for the actual ability score, with most mechanics using the modifier instead.
In the official rules, the modifier is used directly to adjust hit rolls, skills checks, saving throws and other d20 rolls. It is also used directly to adjust damage, uses of turn undead and other quantities.
Ostensibly the score itself is used to determine bonus spells, but since this table is in the same 2-point increments as the modifier table, you could substitute the ability score column with a modifier column without changing the rest of the table.
There are also a number of adjustments and changes that apply to ability scores. However, the most common of these (racial adjustments and enhancement bonuses) always (to my recollection, in edition 3.5) work in even numbers - and any 2-point change to an ability score is equivalent to a 1-point change in ability modifier. So any change to an ability score by an even number of points can be replaced by a change to the bonus of half the size, with no difference to the effect of the rule.
That leaves the following cases where odd ability scores, or changes in ability scores by odd numbers, are relevant in the official rules:
I think I should be able to handle these just by halving the numbers/dice and rounding up or down as necessary.
You can substitute the relevant modifier for the score - for example, +1 for a score of 13. This makes the prerequisite slightly less demanding so there may be cases where it is more appropriate to go up to the next higher modifier.
This rule no longer applies - it is superseded by the potential for characters to gain mind and/or body levels during their careers (see my explanation of, and links to, the natural stats rules).
I haven't figured out aging rules yet - I don't think it will be necessary for PCs for a while. Mental ability increases can be handled by gaining mind levels during a character's lifetime. I may need rules or guidelines for reducing body characteristics by age, which would reduce physical stats, perhaps reducing a character's level.
I anticipate testing rules that replace the minimum-ability-per-spell-level with a system of skills and/or feats. If that doesn't work and I go back to something similar to the official rule, I can simply have a minimum-bonus-per-spell-level rule.
If there's anything else I haven't thought of, the general rule is for a specific score to use the corresponding modifier, and for a change in score, to halve the number and use that as a change in modifier.