As a shieldbearer, you are a warrior of metal and grit. You are a mobile fortress, dedicated to shrugging off blows and defending your allies in harm’s way. You are capable of turning mundane shields into powerful battering rams, and can single-handedly make way for your allies to turn the tide of battle.

Edit: Fixed Shield Bash to not be a bonus attack and belong to the Shield Bash ability, not Improved Protection. I'm not as adept at using this homebrew tool.

Level 3: Improved Protection

When you choose this archetype at 3rd level, you gain the Protection option from the Fighting Style class feature. If you already took that option, you can instead choose a different one. 

Your version of the Protection fighting style has a range of 10 feet, instead of 5, provided that your speed isn’t 0. When you use your reaction in this way, you can move up to 10 feet to a space within 5 feet of the attacked target, which doesn’t provoke opportunity attacks. Even if the triggering attack hits, the damage the target takes is reduced by an amount equal to 1d4 + the shield’s total bonus to AC. This damage is reduced by an additional d4 when you reach certain levels in this class: 7th level (2d4), 10th level (3d4), 15th level (4d4), and 18th level (5d4). 

In addition, you gain a special reaction that you can take on any creature’s turn, even if you’ve already used your normal reaction, but no more than once per turn. You can use this special reaction only to use your Protection fighting style, and you can’t use it on the same turn that you take your normal reaction. You can use this special reaction twice, and you regain all expended uses of it whenever you finish a short or long rest. 

Level 3: Shield Bash

Also at 3rd level, you can use a shield you’re holding to make melee weapon attacks. You are proficient with it, and you add your Strength modifier to its attack and damage rolls. Its damage is bludgeoning, and its damage die is a d6. If your shield grants you a magical bonus to AC, such as a +1 shield, that bonus is also applied to any attack and damage roll you make using the shield in this way. 

In addition, shields have the thrown property for you (despite not being considered weapons), and they have a normal range of 20 feet and a long range of 60 feet. You can also don or doff a shield using a bonus action, and if you throw one you’re wielding in this way, you can doff it as part of the attack. Lastly, if you’re wielding only a shield and no other weapons, your shield is considered to be a weapon for you, in addition to being armor. 

Level 7: Battering Ram

At 7th level, you can use your shield to force your way through any obstacle. When you make a Strength check to break down a door or similar barrier using your shield, you gain a bonus to the check equal to 2 + your shield’s total bonus to AC. 

In addition, when you move at least 20 feet in a straight line toward a Large or smaller target and then immediately hit it with an attack, that target must also make a Strength saving throw. The DC is equal to 8 + your proficiency bonus + your Strength or Dexterity modifier (your choice). On a failed save, the target is knocked prone and takes 1d6 bludgeoning damage. If the target is an object or structure, the extra bludgeoning damage is doubled. You must be wielding a shield. 

Level 10: Active Defense

Starting at 10th level, you can ready yourself to leap into defensive action. Your Protection fighting style can now be used to impose disadvantage on an attack roll made by a creature attacking you, instead of only your allies. When an attacker misses a target as the result of you using this Fighting Style option, you can immediately make one melee attack with your shield against the attacker as part of that reaction, provided that the attacker is within 5 feet of you. You can also use the special reaction from your Improved Protection feature three times between rests, instead of twice.

Level 15: Master of Protection

By 15th level, your Protection fighting style has a range of 15 feet, instead of 10, and you can move up to 15 feet when using your reaction in this way, instead of 10. 

You can also use the special reaction from your Improved Protection feature four times between rests, instead of three times.

Level 18: Impenetrable Taunt

At 18th level, you can use an action to let loose a raucous taunt. Each hostile creature of your choice within 30 feet of you must make a Wisdom saving throw, provided that it can hear you. The DC is equal to 8 + your proficiency bonus + your Strength or Dexterity modifier (your choice). On a failed save, a creature falls for the taunt. An affected creature remains taunted for 1 minute or until it takes damage from a creature other than you. 

A taunted creature has disadvantage on attack rolls against targets other than you for the duration. For each creature affected by the taunt, you gain a +1 bonus to AC, up to a maximum of +5. A taunted creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Once you use this feature, you can’t do so again until you finish a short or long rest.