Please note that this list will probably constantly be updated, so check it every now and then.
Below is a list of some abilities / skills I though of in my spare time. P for Passive and A for Active.
Durability (P):
Increases the player's Max Health by their total health / 11. That is, if you have 12,000 health and equip this ability, you'll still have 12,000 health when you play a game but will be able to increase it to a maximum of 13091 if you pick up or use a hp pack or rb pack.
Triple Shot (A):
While this skill is equipped, the player can press X + C and pay health equal to their max health / 31x to fire three bullets (they lose three bullets instead of one) at once in the same direction, thus dealing triple their normal gun damage.
Power Over Parts (P):
While this skill is equipped, the player deals extra transed and untransed damage equal to the normal + 3% at the cost of losing health equal to 0.6% of their max health every second.
Power Over Parts II (P):
While this skill is equipped, the player deals extra transed and untransed damage equal to the normal + 3% and increases their Critical Hit and Gun damage by 3% at the cost of losing health equal to 0.6% of their max health every second.
Meteor Storm (A):
[see thread "New guns - Flamer and Judgment Day". This ability will use Judgment Day's Area of Effect effect to damage multiple enemies when X + V is pressed]
Spiked Armour (P):
When an enemy attacks you, they lose HP equal to 5 - 7% of the damage they dealt to you (the II version of this could inflict more damage).
Mechanism Reconstruction (P):
If you are stunned, you are only stunned for half the time you would normally be.
Distractor (merc skill):
Gives your mercenary a chance to make all viruses within a certain distance attack the equipped merc. This is a useful 'Taunt' skill and may save the player.
Stats may change to better suit the game.