I'd previously flirted with Astonishing Swordsmen and Sorcerers of Hyperborea here, taking some of the interesting class abilities out of AS&SH and making them ACKS proficiencies.
Back in the combat chapter of AS&SH there's a section on Advanced Combat Actions, and it may be interesting to see how they'd interact with ACKS, when possible (and where they don't interfere with existing ACKS rules)
Some of the stronger ones I've locked behind a proficiency wall. Others get better if you have a related Fighting Style proficiency.
As a quick clarification, these aren't meant to be proficiencies, just advanced combat maneuvers for either any combatant or extra options for proficiencies already taken, or perhaps possible answers to "What if I ..." sorts of things players come up with.
Arrow Setting: The archer sets his arrows into the ground before him, angled towards him for ease of pickup, in order to gain a faster rate of fire. When the archer has taken the necessary preparation time to do this, he may cleave with a composite bow or short bow up to 8 times, and up to 6 times with a longbow, double the usual ability. The archer cannot move away from his set arrows when taking advantage of the set arrows.
AS&SH uses multiple attacks per round, Cleave is ACKS' replacement for that.
Characters with Fighting Style: Weapon and Shield take only a -2 penalty to the immediate attack after a successful bind.
Neither combatant enjoys their shield bonus to AC while the shields are bound. The character may hold the shield bind for a number of rounds equal to their Cleave ability. The opponent may withdraw to break the bind, and the binding character must follow to hold the bind.
As Archaeopteryx rightly points out below, this one did come out a bit overly useless. After further review, there's no real reason this can't be covered by Wrestling from ACKS Core.
Dodge: In lieu of any other non-movement action, a character with Swashbuckling (or similar proficiency) may focus on avoiding enemies' attacks, gaining a +2 to AC. If combined with a fighting withdrawal, the withdrawing combatant may not attack a following enemy.
Double Arrow Shot: A character with Fighting Style: Missile may choose to nock and shoot two arrows at the same time at an opponent within 30 feet. This attack is made at a -2 penalty. If the result is a miss, both arrows miss, if a hit, both arrows hit.
Off Hand Weapon Parry: A character with Fighting Style: Two Weapons may forgo his +1 to-hit bonus for a +1 to AC. This bonus does not count against missile or other ranged attacks.
Parry and Block: In lieu of any other non-movement action, a character with any melee Weapon Style may focus on parrying and blocking incoming attacks with their own weapon and/or shield. This provides a +2 AC bonus (which only counts against missile or other ranged attacks if the character also holds a shield). In addition, characters with a Strength bonus may apply that bonus to their AC. If combined with a fighting withdrawal, the withdrawing combatant may not attack a following enemy.
If an attacker scores a natural 20, the character's weapon or shield is sundered. (magical weapons and shields may only be sundered by magical weapons greater than or equal to their own).
Reckless|Conservative Fighting: A character may fight recklessly, with less concern for his safety, gaining a +1 to-hit in exchange for a -2 to AC. Alternatively, they may fight cautiously, gaining +1 to AC while taking a -2 to-hit. This may not be combined with Berserkergang or similar.
Recumbent Fire: A character may lie flat on his back to take a shot. Archers or slingers must fire off to the opposite side of their handedness (to the left if right handed), crossbowmen are not restricted. Attacks are made at a -4 penalty, or -2 if the character has Fighting Style: Missile. The character is considered prone, but gains a +2 to AC vs. missile fire.
Saddle Fire: A character may fire a ranged weapon from horseback, while the horse is in motion. A walking horse has no penalty. A trot causes a -1 penalty; canter -2, and full gallop -4. A character with Fighting Style: Mounted (from the link above) halves all penalties (no penalty for a trot).
A walking horse is moving at one-half their current (based on encumbrance or other situational modifiers) movement rate. A trot is their regular rate, a canter twice that, and a gallop three times their movement rate.
Shield Cover: A character may forgo any bonus to AC from his shield or Dexterity, and instead provide those bonuses to an ally. This ability may be used to defend an ally who is withdrawing past the character.
Shield Wall: Three or more large shield users may overlap their shields, staying in a tight formation. They each gain a +2 to AC vs. melee attacks, and a +4 vs. missile attacks. In formation, they may attack with one-handed piercing weapons. A second rank of combatants (of equal or lesser number) may form behind the shield wall, and they may attack with long piercing weapons through the wall at a -2 penalty.
This is the same bonus from the defend action in Domains at War: Battles.