Stars Without Number
Rather than have armor determine your AC, characters AC is based on their level – an innate measure of how cool, lucky, and skilled they are. Calculate your AC according to the following formula:
(10 – character level)
Once you reach level 10, your “base” AC can no longer improve in this fashion, and stays at 0. Dexterity and Shields modify your AC as normal, so the best possible AC attainable is -3 for a 10th level character with 18 dexterity and a shield.
Design Note: This system makes characters innately harder to hit, and less dependent on armor. On the flip side, it becomes difficult to increase AC by use of equipment, and armor instead provides extra HP for more survivability.
Armor as HP
Rather than negate the possibility of hits, Armor has a HP pool of its own, allowing it to soak some incoming damage. You always lose these hit points first in a given fight, though they reset automatically at the end of combat. In order for armor to be as useful for high-level characters as it is for low-level characters, armor scales with level.
Unarmored: No extra HP. Completely unarmored and unprotected characters are vulnerable to the slings and arrows of outrageous fortune.
Leather Jack/Armored Undersuit: If your armor would normally provide 7 AC, it is on this level. Leather Jacks are ineffective against high-tech attacks, and against such, your armor HP does not apply. Armors of this type have 1 HP per level.
Metal Mail/Woven Body Armor: If your armor would normally provide 5 AC, it is on this level. Metal mail is ineffective against high-tech attacks, and against such, your armor HP does not apply. Armors of this type have 2 HP per level.
Metal Plate/Combat Field Uniform: If your armor would normally provide 3 or 4 AC, it is on this level. Metal plate is ineffective against high-tech attacks, and against such, your armor HP does not apply. Armors of this type have 3 HP per level.
Assault Suit/Deflector Array: If your armor would normally provide 2 AC, it is on this level. Armors of this type have 4 HP per level.
Powered Armor/Field Emitter Panoply: If your armor would normally provide 0 or 1 AC, it is on this level. Armors of this type have 5 HP per level.
Design notes: Armor granting extra HP, scaling with your level, continues to make armor a tremendous boon even for high-level characters. I opted for it in place of damage reduction because it allows a skilled character with a weak weapon to take out heavily-armored opponents, albeit at a disadvantage. With the DR system, this either became impossible or needlessly clunky.
If this variant rule makes fights take too long because of the increased HP, considering letting characters add their Skill ratings to damage as well as to attack, if they have the Skill. For the Unarmed skill, add twice the Skill rating to damage. This ups damage, further incentivizing the use of armor in combat despite the innately better AC scores.