Here's my example of Ability Damage Tracks.
I selected three tracks (BODY, MIND/AGILITY, & HEART) because any more than that becomes unwieldy for the GM to track with monsters or foes. Condensing it to 2 (MIND/BODY) like C&C does would also work.
Each track can be in one of six states:
Extraordinary, Superior, Normal, Disrupted, Degraded, Destroyed.
Feel free to reskin the names if you want. For example, with morale, it could be "Impetus, Brave, Normal, Shaken, Frightened, Panicked."
A track can be raised to a Superior state at character creation using a Prime system a la C&C. You could also use my "focus" system (described earlier) to allow players to dynamically shift it between encounters. Having a Superior or Extraordinary track gives minor benefits, bonuses to related checks, and gives you more of a buffer before that track is destroyed.
Various attacks might temporarily lower a track and buffs might temporarily raise it. If a track is not in its "natural state," either from a penalty or a buff, then each turn the player flips a coin for each abnormal track. There's a 50% that the track moves one step towards its natural state.
ADVANCED RECOVERY RULE: Make a "saving throw" for each track that is not on its normal resting state. This save is a body check. If a track has been penalized, a successful check moves the track one step towards its natural state. If a track has been buffed, a failed check moves the track one step towards its natural state. Note that this rule makes a "tail spin" possible, in that once your Body attribute is targeted, it becomes more difficult to recover from ability score damage.
Note that the tracks are designed so that being at the "destroyed" level will not necessarily render you combat ineffective, but may. For example, being immobilized might render you useless.
An effect could hit multiple tracks at once. For example, a spell that stuns its foe might afflict them with -3 BODY and -3 MIND.
BODY (Vigor) - Holds, Endurance
- Extraordinary: Checks +1*, +2 hexes speed
- Superior: Checks +1, +1 hexes speed
- Normal: Checks +0, Normal speed
- Disrupted: Checks -1 on D20, 1/2 speed
- Degraded: Checks -3 on D20, speed max 2 (or 1/2, whichever is lower)
- Destroyed: Checks -6 on D20, immobilized
MIND (Agility, Reflex) - Beguiling, Initiative
- Extraordinary: Checks +1*, 1 std/1 move/3 minor
- Superior: Checks +1, 1 std/1 move/2 minor
- Normal: Checks +0, 1 std/1 move/1 minor
- Disrupted: Checks -1, 1 std/1 move/0 minor
- Degraded: Checks -3, 1 std/0 move/0 minor
- Destroyed: Checks -6, 0 std/1 move/0 minor
HEART (Wisdom, Charisma) - Senses, Morale
- Extraordinary: Checks +1*, D12 Fate Die
- Superior: Checks +1, D10 Fate Die
- Normal: Checks +0, D8 Fate Die
- Disrupted: Checks -1, D6 Fate Die
- Degraded: Checks -3, D4 Fate Die
- Destroyed: Checks -6, D1 Fate Die
*Fate dice are basically an extra die added to a D20 check. They allow players to control the odds to some degree. Reducing a player's control over the odds is likely to make them more cautious and fearful of getting in over their head, the desired effect. For NPCs, a morale check impact might be more appropriate.
One thing I am not sure about are the numerical bonuses and penalties. First, OD&D uses D6s for most subsystems. Giving +1 for a superior/extraordinary trait is viable, especially if it largely subsumes a skill system. But giving a -2 penalty means that characters may not be able to succeed at all with an untrained skill.
For example, say you have a system where players -- in general -- succeed on a 4-6 (rolled on a D6). If they are superior/extraordinary, then they succeed 3-6. If they have a -1 penalty (degraded), they succeed 5-6. And if they have a -2 penalty, they succeed only on a 6. Then again, that's probably ok -- someone with a Destroyed attribute should suck a whole lot. If you've been reduced to a quivering mass of cowardly jelly by a fear spell, your attempt to rally the troops is likely to be unsuccessful. Likewise, if you've been hit by a nasty Ray of Enfeeblement, your attempt to win any physical contest will be very limited indeed.