I've Gone around and collected some suggested abilities from lots of different posts,   Both to compile them, and were possible/needed describe how they would be achieved in game using my suggested system.  Suggested system found at link.

Please suggest other Abilities, I will attempt to edit them in and implement them into the  'attack, damage, def,  armour, targets and swings' model.

Fly, Can only be hit with Ranged Weapons, and Counter Attacks.
 

Walking allows better movement over Certain Terrain types. (Combined with Charge can prove deadly).
 

First Strike, When Attacked will Swing first.

Defender, Will not Strike back when attacked but gets 25% Def .

Last Strike, Will Attack Back even if Killed, may also be liken to units that when killed explode or acid for blood.

Cause Fear, Unit unsettles Foes, Attackers will get -20% Hit Dam, and DEF unless, makes Morale Roll. This would allow different levels of Fear say Base for Skeletons, to Terror Morale Roll -30 Points for Big Demons and Dragons.  (if the Roll is bad enough unit might route)
Would also give more Weight to Morale Rules.

Stealth,  Make a check to see the unit, if Missed you know that it is there somewhere, but Ranged attacks are mostly useless -75% to Hit -50% to Dam, and Until they attack even Melee is somewhat hampered -20% to Hit and Damage. Area Effect spells not Effected.

    Implemented by:  If not seen reduces number of 'targets' in unit to 0.5 when attacked by archery, (half damage), First attack Roll is calculated like a re-roll, with no further rerolls (effectively -75% to hit, and another - to damage,  might use '.75' of a target so that the combined is -50% damage)

Shield Other, Allows a unit to body guard another, If Ranged Attack Body Guard has chance to take the Damage instead, but Damage is Increased by +25%. Melee would work in the same manner with Damage only +10%
An Advanced Version of Skill might allow Magic attacks to be intercepted as well.

      Implemented by: Swings directed at Unit B Hit a target of A (if a full target exists) with +% damage.  If the 'swing' Misses the first target, rerolls on either Unit A or Unit B (by comparing how many 'free' targets remain on both units.)  [Optionally increases the number of 'targets' on A by 0.5-1.5, because they are actively seeking to get hit].

Cavalry fend- Gives a defensive bonus when attacked head-on by cavalry.

      Implemented by: Def on primary target of 'swing' increased, no secondary rolls for miss of first.  Damage increased on counter attack.(but not attack, so no extra ability to hit, but extra damage if you hit because of cavalry speed.)

Skewer- Gives attack bonus on the charge. 

Unwieldy in melee- Gives a melee attack penalty unless vs. another spear. If unit in melee with enemy and flanked by other enemy unit, the spearman recieves a penalty to defense.

      Implemented by: no secondary re-rolls for units with Unwieldy.

Thrown- The spearman may throw the spear but if he does he loses it until he enters the square where it landed. As the spear was not designed for throwing there is an attack penalty for doing so.

 FORMATION

Cantalabrian Circle ... (Only Horse Archers; reduce effect of enemy archer fire)

      Implemented by: 'Targets' per unit in the circle reduced by .5, incoming archery 'Swings' have additional 50% chance to miss(they still reroll on secondary attempts as usual), 

Tight/Box

Implemented by: Can only use 2/3rs of available swings, but only has 1/2 of the available targets (when attacked by melee), Archery attacks consume 1/4 of normal 'targets' when hitting.

Wedge, however theoretically melee armed cavalry would be the best at it.

      Implemented by: When attacking 'tight' formation if succeeds number of available Melee targets is 3/4 normal, rather then just half, rerolled swings reroll as primary swings rather then reroll swings. Condition permanent until dis-engaged. Must kill at least (1/6th) of units in 'tight' formation with first attack to succeed. 

loose, good against archery

     Implemented by: incoming archery like swings only get one reroll rather then however many they normally get. (determined by archery type) This formation has all swings and all targets, also good for attacking squads that are smaller then you, to not reduce your 'swings' count.