Rogues should get bonus AC, using some formula that scales with increasing Dexterity and XL.

Artifact helm of telepathy, neutral aligned, that either enhances or blocks telepathy level-wide based on whether you have it from some source other than the helm (or alternatively, it’s not a helm of telepathy, and just bases things on whether you are telepathic), and grants a Charisma-based bonus to your armor class.

If you do have telepathy, all other monsters get telepathy too and you can sense them, and they will be affected by telepathy-targeting effects, including yours if you have any psionic abilities.

If you don’t have telepathy, all other monsters will be treated as if they are not telepathic, even if they are; they will also be blocked from using any mind blasts or other psychic powers.

Add a system of “hell curses”. When you enter a demon lord’s lair, you are afflicted with 2 random curses, which stick around forever until the lord is killed or bribed off.

You can receive at most six curses over the course of the game, requiring you to defeat the lords of the first three lairs you encounter. Further generation of lairs does not add any curses.

A non-comprehensive list of potential curses is:

• healing potions heal only half as much HP
• your speed is knocked down by 2
• your AC is deducted
• your spellcasting failure rates increase
• you are randomly inflicted with confusion every 50 turns or so

Various effects for armor pieces based on their randomized description:

• Buckled boots and old gloves: cost (and sell for) twice as much in a shop.
• Hiking boots: prompt before walking into known pits or holes.
• Mud boots: autocurse when worn.
• Padded gloves and visored helmet: +1 to AC.
• Tattered cape: -1 to Charisma.
• Ornamental cope: +1 to Charisma.
• Opera cloak: some sort of bonus when using musical instruments.
• Plumed helmet: +1 to Charisma, -1 to Dexterity (or have these adjustments multiplied by its enchantment).

Adjust the AC values of dragon scale mail (or possibly, the maximum amount to which they can be enchanted) to even out their general utility to the player. Specifically:

• Ones that give powerful extrinsics such as silver and gray should have their AC substantially nerfed.
• Really underutilized ones that give redundant common extrinsics such as green and orange should have their AC raised.

In order to penalize situations where the hero is partly or mostly surrounded without implemeting a complex flanking system: Every monster attack that incurs damage reduction due to subzero AC makes your effective AC worse by the amount of reduced damage for the duration of that turn. This means that a crowd of monsters hitting you all at once will whittle down your defense and do some damage rather than damage reduction being in full force for each individual attack.

When you look at a pet’s inventory via the #loot command and it is wearing armor, its AC is shown.

In variants that print messages such as “Foo’s [armor piece] deflects your attack”, extend this system to monsters that have relevant defensive body parts even if they’re not wearing armor. Do this by crediting them with an appropriate amount of fake AC when determining what string to use for “[armor piece]”. Then players that miss monsters instead get messages about the monster’s natural armor preventing them from doing any damage.

The inspiration for this is EvilHack’s giant turtle, whose AC of -8 prompted many inexplicable “miss” messages for such a large and slow creature.

If you aren’t wearing armor or a shield and have high martial arts/bare hands skill, you get an AC bonus (probably a higher one for martial arts). The bonus should probably scale up with higher skill, and should be generally higher for martial arts users than bare hands users.

Damage reduction from AC happens before half physical damage instead of afterward.

AC you have from your polyform should not stack with AC you have from armor. The game should compute both, take the max, and then add on AC from other factors like protection.

Being very fast reduces your AC and/or to-hit.

Yendorian Army units surrounded by fellow army units have higher AC, to-hit, and damage.

Rebalance armor so that all “mail” armor and most of the iron body armor in general is not useless. Ideally, the worst iron armor (orcish chain mail) should be considerably better than studded leather, but they are equivalent. The ideal balance, keeping leather armors and all weights constant and not accounting for to-hit or damage reduction, is probably something like the mails going up to 9 or 10 AC, and splint and plate getting up to around 14, but this is not possible to implement without changing the combat system.