HP and HP
The HD and HP idea comes apes the Flesh and Grit mechanic of another OSR ruleset I forgot the name of, but I learned about from Cavegirl’s own OSR game. But, I’m going to complicate it a bit by adding Scarlet Heroes’ HD damage table.
Some attacks damage HD directly. Most attacks, however, must deplete HP first. Zero HD is death. Zero HP is loss of combat effectiveness. HD can only be replenished through major medical attention. Damaged HD will also the number of dice used for HP accordingly. HP is replenished through rest and eating, or minor medical attention.
Separating HD from HP also makes it a simpler thing to use that other mechanic I found out about concerning re-rolling HP whenever a party rests and eats, with good quality rest and eating choosing whichever roll is highest, and so on.
And as for CON bonuses, such bonuses will only apply as extra hits to one’s HD, not extra dice.
So, in summary: HD+Hit Bonus is separate from HP. Only HD creates HP.
AC and AP
With the same idea as above. AC creates Armor Points. Armor points act as Damage Soak and Durability for the armor and will make equipment maintenance much more present in games.
Primarily, the rules here are for ascending DC mechanics.
So, AC will serve as a base for AD, or Armor Dice. Armor Dice will be based on quality, ranging from poor quality D4 armor all the way up to legendary d20 armor. AC will also determine how many of said AD will be thrown to determine AP.
Before an attack could hit HP, it must first deplete AP. But, and attack from behind ignores AP. If an enemy hits, the attack goes straight to HP.
So, in summary: deplete AP first to deplete HP, then deplete HP to deplete HD.
Repairing armor allows you to re-roll your AP, but Zero AP means the armor is salvageable.