I got the idea from the "Damage per distance" thread that go no'ed. It would be something I personally could make use of as well for balancing some of DE's weapons.
Originally, I was going to suggest a generic function that change any property; such as "A_ChangeProperty(flag "Property", bool add(1)/subtract(0), int amount)) but that seemed too broad and probably cause some issues.
A_ChangeDamage would be similar, although it will only affect the damage property for the actor. The parameter field are as follows:
A_ChangeDamage(in amount, bool add/subtract)
Each time the function is called, the set amount will be added/subtracted to the damage property of the actor. Thus a modder can subtract damage from a projectile if after the missile has flown a certain distance, or weaken a monsters melee damage after it's been injured. Obviously, the function should check to see if the damage is already at 0 to avoid negative damage values.
Maybe a scaling parameter can also be added, multiplying the damage by a float scaling factor after the initial adding/subtracting.