Armors Revised

Penalties

Non-Proficiency

While wearing this Armor without Proficiency, you have Disadvantage on Strength and Dexterity Ability Checks and Saving Throws, Disadvantage on Attack rolls, you can't provide Somatic Components (such as for Casting Spells), and you gain a -10ft Speed Penalty (to a minimum of 5ft).

Minimum Strength

If you do not equal or surpass the Minimum Strength Score required, you do not count as Proficient with this Armor.

Mitigation and Resistances

In D&D5e (PHB pg.197), it is explained that you first add any Bonus and Modifier to a Damage roll (such as Damage reduction or increase) and after you apply any Resistances and Vulnerabilities.

Shield Proficiency

To be Proficient with a Shield, you must have both Proficiency with Shields, and Proficiency with the appropriate type of Light, Medium or Heavy Armor, as shown on the Shields Revised table. The Rogue is a special case:

1st-Level Rogue. When you take your 1st Class Level in the Rogue Class, you gain Proficiency with the Buckler Shield.

Shield Attack

While wielding a Shield with Proficiency, you can use it as a Simple Melee Weapon to make Melee Weapon Attacks with the Damage dice shown on the Shields Revised table.

If you are Proficient with the Shield you wield, you are also Proficient with it for your Shield Attacks.

Grip

The Grip of your Shield determines the Actions you need to take to equip and wield it.

Handle. You can Don or Doff this Shield as an Interaction, and you can drop it automatically as a Free Action. If someone tries to Disarm you of this Shield, they have Disadvantage on their Disarm roll.

Strap. You can Don or Doff this Shield by replacing one Weapon Attack or as a Bonus Action. You cannot be Disarmed of this Shield when you wield it while conscious.

Penalties and Minimum Strength

Shields count as Armor for the purposes of the Penalties you receive when wielding them without Proficiency, and lacking the Minimum Strength Score gives you no AC Bonus.

Shield Properties

Kite. The Kite has the shape of a drop of water, being favored by the cavalry. When you wield it while mounted on a Mount in which you are Proficient, your Mount also benefits from the AC Bonus of the Kite Shield.

Pavise. The Pavise is a tall and oblong Shield, providing portable Cover. When wielding it, as an Action you can deploy the Pavise within one of the borders of your space, Doffing it off. In this state, it stands on its own as a wall, granting Half Cover to Creatures its Size standing within its Space and behind the border wall, or Three Quarters Cover if Prone (it has 15 AC, 20 HP, and if reduced to 0 HP its destroyed). A Creature within 5ft can remove the Pavise from its deploy state by replacing one Weapon Attack or as a Bonus Action.

One Shield at a Time

As per the PHB page 144, you can benefit from only one Shield at a time. So even if you wield one Scutum in each hand, or a Scutum and a Pavise, their AC Bonuses and benefits/rules do not stack, you choose which one to keep.

Aspis. The Aspis is big and round, allowing you to reposition it for Cover. When wielding it, as a Reaction when you see yourself or a Creature within 5ft of you being taken as the Target of an Attack or Effect, you can give yourself or the Creature within 5ft of you Half Cover against the Attack or Effect, except against Effects that move around corners.

Scutum. The Scutum, sometimes called a Tower Shield, is big and heavy, allowing you to Cover almost your whole body. When wielding it, you count as Three-Quarters Cover for Creatures of your Size; instead of just Half Cover. In addition, as a Reaction when you see yourself being the Target of an Effect, you gain Three-Quarters Cover against the Effect, except if it can move around corners.

While wielding the Scutum, you cannot make Attacks with Weapons that lack the Light Property, unless you have the Shield Master Feat (PHB page 170).

Dueling Shield. The Dueling Shield is a big rectangular Shield with large triangular hooks and blades built into the rim. This Shield counts as a Martial Melee Weapon.

Mitigation

New Mechanic
Mitigation reduces the total amount of certain Damage types a Target takes since the start of a Turn until the end of that same Turn (to a minimum of 0 Damage, and on such cases treating the Attack/Effect as a Miss).

This reduction applies to the total Damage taken in a Turn, even if its from multiple Attacks, Effects or Damage types (you can never reduce more Damage per Turn than your highest Mitigation number in total).

Multiple sources of Mitigation confer it against different Damage types, but the Mitigation number does not stack with other sources of Mitigation; you keep the highest one of each Damage type.

Weakness

New Mechanic
Weakness increases the total amount of certain Damage types a Target takes since the start of a Turn until the end of that same Turn.

This increase applies to the total Damage taken in a Turn, even if its from multiple Attacks, Effects or Damage types (you can never increase more Damage per Turn than your highest Weakness number in total).

Multiple sources of Weakness confer it against different Damage types, but the Weakness number does not stack with other sources of Weakness; you keep the highest one of each Damage type.

Exchange AC and Mitigation

New Option for Monsters & GM
The GM can increase/reduce the AC of a Creature in +/-1 in exchange for reducing/increasing its Mitigation in -/+2 or vice versa. The GM can do this multiple times, but a Creature's AC cannot be lowered below 10 + its Dexterity Modifier, nor its Mitigation be reduced below 0.

This is an option that allows the GM to make/modify Monsters to have a mechanical difference between fast and dodging foes that avoid an Attack with high AC (such as a Deva), from slow and sturdy enemies that tank an Attack with high Mitigation (such as Dragons).

Only Monsters. This option is only for NPCs/Monsters, it is not meant for Player Characters as they are already balanced with the AC and Mitigation they have.

New Feats