This is all based off of one change to flatfoot that actually doesn't change it up too much, but compliments one of the skills suggested, that change is:
-If flatfoot is applied to a target with flatfoot already applied, then they are knocked down instead. (this is basically non-consequential, and helps charge-bash users a lot, removing the RNG for spending way more momentum)
Anyway here's my suggestions:
Pin (Black Knight) - Shield only offensive skill
This skill can only be used with a shield equipped, targets 1 enemy in 1 range and smashes them with your shield, dealing blunt physical damage equal to twice your Scaled STR + a bonus based on rank (10/20/30/40/50), this skill has a status infliction based chance to immobilize for 2 rounds, if the target is flatfooted then the duration is increased to 3 rounds.
Having Enchanted Shield active will cause shield bash to deal magic damage instead, changing its damage type to its element.
Gambit (Black Knight) - Shield only offensive skill
Lob your shield to a target within 5 range (think hanging targetting) which will deal blunt physical damage equal to twice your Scaled STR + a bonus based on rank (4/8/12/16/20), this will also inflict flatfoot LV = (Rank*3).
Having Enchanted Shield active will cause Gambit to deal magic damage instead, changing its damage type to its element.
Thats all, thank you for your consideration.King's Guard (Black Knight) - Shield only equipable utility skill.
Passive: Divert attention onto yourself, make yourself the center of the blast, when an AoE attack is used on you, allies within 5 range (Circle) also targetted by the same AoE gain 20% Damage reduction for that damage instance, only 1 King's Guard takes place at any time.
Active (3 round cooldown): Spends all of your momentum and bunkers down, completely immobilizing you and granting you knockback/pull immunity, as well as (Momentum spent * 10) LV Guard, allies who are considered behind you within King's Guard's radius also gain triple its DR during this.
(NOTE: King's Guard is not intended to apply to the user, only their allies)