Terath Ninir
Yog Sothoth loves you
This idea just popped up in my head today when I was flipping through my PHB. I haven't seen many feats or class abilities to improve armour, and I haven't liked the ones I've seen.
I'm aware that these feats, as presented, might be a little on the powerful side. In their defense, however, I'd say it's kind of necessary. D&D is such an offense-centered game that any defensive abilities are going to have to be pretty darn spiffy to get chosen. These are going to have to compete with improved critical and cleave; a small bonus to armour just flat out isn't going to get chosen, I think.
Armour Focus [General]
Choose one category of armour (light, medium, or heavy). You are especially good at using that type of armour.
Prerequisites: Proficiency with selected type of armour, base attack bonus +1.
Benefit: The maximum Dexterity bonus for your armour is increased by 2. The ability check penalty is decreased by 2, to a minimum of 0. The arcane spell failure chance is reduced by 10%, to a minimum of 0%. You can also don your armour more quickly, allowing you to gain the full benefit of the armour if you don it hastily.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Armour Focus as one of his bonus feats.
Armour Specialization [General]
Choose one category of armour (light, medium, or heavy). You gain extra protection from that type of armour.
Prerequisites: Proficiency with selected type of armour, Armour Focus with selected armour, base attack bonus +4.
Benefit: You gain a +2 bonus to the rating of your armour. This is considered an armour bonus.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Armour Specialization as one of his bonus feats.
Greater Armour Focus [General]
Choose one category of armour (light, medium, or heavy). You are especially good at using that type of armour.
Prerequisites: Proficiency with selected type of armour, Armour Focus with selected armour, base attack bonus +8.
Benefit, Medium or Heavy Armour: The maximum Dexterity bonus for your armour is increased by 2. The ability check penalty is decreased by 2, to a minimum of 0. The arcane spell failure chance is reduced by 10%, to a minimum of 0%. Your armour is also treated as if it were one category lighter for all purposes, including movement. These bonuses stack with the bonuses from Armour Focus.
Benefit, Light Armour: Your armour no longer has a maximum Dexterity bonus; you gain your full Dexterity bonus to AC, no matter how high your Dexterity is. The ability check penalty is decreased by 2, to a minimum of 0. The arcane spell failure chance is reduced by 10%, to a minimum of 0%. These bonuses stack with the bonuses from Armour Focus.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Greater Armour Focus as one of his bonus feats.
Greater Armour Specialization [General]
Choose one category of armour (light, medium, or heavy). You gain extra protection from that type of armour.
Prerequisites: Proficiency with selected type of armour, Armour Focus with selected armour, Armour Specialization with selected armour, base attack bonus +12.
Benefit: You gain a +2 bonus to the rating of your armour. This is considered an armour bonus. This bonus stacks with the bonus from Armour Specialization.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Greater Armour Specialization as one of his bonus feats.
I'm aware that these feats, as presented, might be a little on the powerful side. In their defense, however, I'd say it's kind of necessary. D&D is such an offense-centered game that any defensive abilities are going to have to be pretty darn spiffy to get chosen. These are going to have to compete with improved critical and cleave; a small bonus to armour just flat out isn't going to get chosen, I think.

Armour Focus [General]
Choose one category of armour (light, medium, or heavy). You are especially good at using that type of armour.
Prerequisites: Proficiency with selected type of armour, base attack bonus +1.
Benefit: The maximum Dexterity bonus for your armour is increased by 2. The ability check penalty is decreased by 2, to a minimum of 0. The arcane spell failure chance is reduced by 10%, to a minimum of 0%. You can also don your armour more quickly, allowing you to gain the full benefit of the armour if you don it hastily.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Armour Focus as one of his bonus feats.
Armour Specialization [General]
Choose one category of armour (light, medium, or heavy). You gain extra protection from that type of armour.
Prerequisites: Proficiency with selected type of armour, Armour Focus with selected armour, base attack bonus +4.
Benefit: You gain a +2 bonus to the rating of your armour. This is considered an armour bonus.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Armour Specialization as one of his bonus feats.
Greater Armour Focus [General]
Choose one category of armour (light, medium, or heavy). You are especially good at using that type of armour.
Prerequisites: Proficiency with selected type of armour, Armour Focus with selected armour, base attack bonus +8.
Benefit, Medium or Heavy Armour: The maximum Dexterity bonus for your armour is increased by 2. The ability check penalty is decreased by 2, to a minimum of 0. The arcane spell failure chance is reduced by 10%, to a minimum of 0%. Your armour is also treated as if it were one category lighter for all purposes, including movement. These bonuses stack with the bonuses from Armour Focus.
Benefit, Light Armour: Your armour no longer has a maximum Dexterity bonus; you gain your full Dexterity bonus to AC, no matter how high your Dexterity is. The ability check penalty is decreased by 2, to a minimum of 0. The arcane spell failure chance is reduced by 10%, to a minimum of 0%. These bonuses stack with the bonuses from Armour Focus.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Greater Armour Focus as one of his bonus feats.
Greater Armour Specialization [General]
Choose one category of armour (light, medium, or heavy). You gain extra protection from that type of armour.
Prerequisites: Proficiency with selected type of armour, Armour Focus with selected armour, Armour Specialization with selected armour, base attack bonus +12.
Benefit: You gain a +2 bonus to the rating of your armour. This is considered an armour bonus. This bonus stacks with the bonus from Armour Specialization.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take this feat, it applies to a different category of armour.
A fighter may select Greater Armour Specialization as one of his bonus feats.