Some ideas that came to mind which might open up interesting new build possibilities (generalist weapon masters for example per Man at Arms)
Tier 1:
Formation Fighter: You gain +3 Resolve, Melee and Ranged Defense if you have an ally adjacent, or for each adjacent ally with this perk (Max +15), whichever is greater.
Skulker: You are hidden while beside cover (other than combatants), and you remain hidden if you kill an enemy while hidden, or if you would begin and end a movement while hidden, ignoring zones of control during this movement.
Night Fighter: The penalty to ranged accuracy and sight at night is reduced by 50%.
Distract: You gain the Distract ability: Cost: 4 AP, 20 Fatigue: Applies a stack of Distract to an enemy. Each stack gives allies a +10% chance to hit the target, and gives the target a -10% penalty to hit all combatants other than you.
Slippery: Your chance of escaping grabs, entangling roots and nets is doubled, and your defense is doubled against attacks of opportunity.
Tier 2:
Shield Parry: While equipped with a shield, gain the Shield Parry ability: Cost: 4 AP, 20 Fatigue. While this ability is active, whenever an adjacent enemy misses you with a melee attack, you immediately use the Knock Back ability against it. On a hit, it gains 10 Fatigue.
From The Shadows: Attacks made against an enemy you’re hidden to or unseen by are guaranteed to hit the head (if they hit), ignore its melee defense bonus (including shields) and deal +20% damage to it. You can only benefit from this perk once per turn.
Tier 3:
Massed Assault: You gain +3 accuracy on attacks against enemies for each attack an ally has already made against them on the same turn.
Thrill of the Kill: Once per turn, when you kill an opponent, you regain 20 Fatigue.
Tier 4:
Shield Mastery: Shield skills build up 25% less fatigue. Knock Back gains +20% accuracy, and each attack deflected by your shield while Shield Wall is active inflicts 5 Fatigue on the attacker.
Master at Arms: Weapon abilities cost 10% less Stamina, and you gain +5% melee and ranged accuracy for each Mastery perk you have, including this one.
Tier 5:
Counterattacker: Each time an enemy misses you, you gain a stacking +10% accuracy and damage bonus on attacks against it until the end of your next turn or your next attack.
Gruesome Flourish: When you kill an opponent, Resolve checks enemies make to avoid demoralization are made at a 50% penalty.
Tier 6:
Charge: You gain the charge ability per the Orcs: Cost 4 AP, 30 Fatigue.
.