A Half Orc, fully equipped in brilliant plate mail rallies his soldiers and heads into battle. he wields only a shield the size of a jail door, thick spiked teeth erupt from the front. He charges the enemy and spears them with his shield. He piles bodies against it, he will not falter for he is the front line. 

A Human breathes a cold breath, the air hangs still, floating like a ghost. She brandishes a worn hatchet and is wearing padded furs and a large circular shield with markings and runes covering the face of it on her back. She sees a battalion, marching up the snowy trail to her tribe. She leaps into battle and immediately starts attacking, with two hands cleaving into the men Th.e soldiers try and strike this woman, however the large shield on her back gives her the ultimate defense, she will defend her tribe to the death. 

An Aaracokra man flies over a city, wielding a small hand crossbow and a sharp Kite shield. He sees a woman being harassed into an alley. He swoops in firing once into the group of men. They retaliate firing up towards him. Dodging and deflecting the bolts he barrels in skewering one of them onto his sharpened shield. He lifts his shield up and narrows his eyes just above it. He must defend those who most need it, who else will?

 

The Great shield Knight is a  amazing defender and asset to the team, bringing a walking fortress onto the battlefield and drawing fire. Great Shield Knights work best when in the thick of it, maybe assisted by a Paladin with an aura or Druid reinforcing them with buffs. The GSK does not necessarily need to be a "knight" like with the human woman, they can just be dutiful defenders or maybe a coward that ended up learning how to bash really hard from behind the cover of a shield. Also using the shield as a "weapon" is not required, the buffs and passives work as long as you have it currently equipped,. but you will lose the benefits if you wield heavy arms or anything that requires both hands or being restrained, immobilized etc. 

Weaponized Shield

At level 3: you have gained the ability to attack with your shield. Your shield does 1d6 damage, you are proficient with it and uses your STR modifier.  With your shield you can use STR  instead of your DEX on DEX based saving throws.

Bulwark

At level 3: You have gained the ability to defend your allies in battle, allies within 5 ft of you have +2 AC and when defending against Dragon Breath Attacks/Fireballs (other DEX based saves) your allies gain advantage on DEX saves if the are within a 15 cone behind you.  You have continued to show yourself as the pillar of strength, 

Shield Expertise

At level 7: You have become aware of your allies needing assistance. As a reaction you can run 15ft feet and defend an ally. You impose disadvantage on your attacker and if they hit, they strike you instead. Also your "Weaponized Shield" shield damage is increased to a D8. 

Walking Fortress

At level 10: You have bolstered your defense, you gain an additional +1 AC when wearing a shield. Your Bulwark ability gives your allies an additional +1 AC and the cone behind you increases to 30ft. You also gain the ability to over power your enemies with your shield. You gain advantage on STR checks/saves while wearing a shield. 

Spell Ward

At level 15: When being targeted by a spell, you may use a reaction to attempt to reflect the spell. Roll a D20 1-10 spell targets you as normal, 11-17 You reduce the damage taken by a quarter (if it has damage) if there is no damage you gain advantage against the spell save, 18-20 you reduce the damage by half, (if there is damage) if there is no damage you automatically save against the spell. You may only use this feature 3 times per long rest. 

Defender Supreme

At 20th level: Your "Spell Ward" now can reflect spells onto the attacker, on 18-20 the spell is absorbed into your shield and is re-cast onto the attacker, using their Spell Save DC. Your "Bulwark" ability increases its effective distance to a 45 foot cone. Your "Shield expertise" when used gives your defended ally temp HP equal to half your fighter level. Finally your "Weaponized Shield" shield damage is now a d10.