Hrum. Depends on how you intend to make it work.
For instance, if PC Fighter A starts off with 4 ranks in Weapon Skill: Longsword, and NPC Orc Warrior B starts off with 4 ranks in Weapon Skill: Greataxe instead, how would they resolve their attacks against eachother? Would they use the skill for whatever weapon they're wielding at the time, such that Fighter A attacks with a longsword using WS:L for his "attack roll" skill check, while Orc B defends with a greataxe using WS:G for his opposed "defense roll" skill check?
What if the defender has no weapon skill ranks, or only has ranged weapon skill ranks? Such as, Archer C gets attacked by Orc B, can Archer C defend with his Weapon Skill: Longbow? What if Wizard D gets attacked while he's unarmed, and has no ranks in Weapon Skill: Unarmed? What if Archer C is shooting Orc B with his longbow, does Orc B defend with his Greataxe skill? What if Wizard D blasts Orc B with a Ray of Frost spell, what skill would the Wizard use for his ranged touch attack roll, and how would Orc B defend? How would the difference between touch attacks and normal attacks function?
How would a character's Dexterity bonus to AC factor into all this? How would armor, natural armor, dodge bonuses, deflection bonuses, shields, competence bonuses, luck bonuses, insight bonuses, and such work? Obviously such things as competence, luck, insight, and dodge bonuses couldn't reduce the damage directly, and ought to only reduce the chance of being hit.
How would the weapon skills and such be set up? Would they be specific like my examples above, or more generic, such as Weapon Skill: Swords, or Weapon Skill: One-Handed Swords, or Weapon Skill: Martial Swords, or Weapon Skill: One-Handed Martial Swords, or Weapon Skill: Martial Melee Weapons, or Weapon Skill: One-Handed Martial Melee Weapons? What would the difference be between proficiency and non-proficiency?
Stuff to think about before implementation.
Here's some stuff I had worked on a few months ago that isn't quite finished yet, but is supposed to be part of a variant on 3E D&D. I'll just copy-paste the relevant parts relating to attack and defense. An important note is that HP in this variant are based on a simple combination of Size and Constitution rather than level, and it's supposed to use a point-buy method for advancement; gaining levels grants more points to spend on character abilities, such as skills. This (as-yet incomplete) system has no classes, and instead uses feats to bestow class-like abilities. More potent/difficult-to-learn feats have higher cost than common feats.
A weapon skill, when gained, grants full proficiency in 1 weapon of that type, with partial proficiency in all other weapons of that type. Points are spent separately, more cheaply than new ranks, to expand partial proficiencies to secondary proficiencies, or on up to full proficiencies. Partial proficiency applies only 1/2 the total bonus of that skill as well as an extra -2 penalty when making a check with a weapon the character is only partially proficient in (i.e. an attack or defense roll), while half-proficiency applies 3/4 the total bonus of that skill instead and incurs no -2 penalty. Combination Weapon Proficiency is a feat that functions similarly to Two-Weapon Fighting, though kinda differently, and can apply to any kind of weapon that can be used in one hand. The Projectile (Arquebus) skill is meant to be for old-fashioned firearms, ones more complex to use/load than modern firearms, while Projectile (Firearms) is for more modern guns, and Projectile (Siege) is for old siege weaponry, while Projectile (Artillery) is for more modern artillery pieces as well as mortar tubes and tank cannons and such. The Throwing weapon skills are pretty vague in definition of the applicable weapons, but for instance Darts applies with all pointy/sharp, reasonably-balanced throwing weapons, Spears is for larger such throwing weapons, Slings applies with bolas and nets and such too, and Shotputs applies with throwing hammers and such too. Skill defaulting is sorta like Shadowrun's skill defaults; if no ranks are possessed in the main skill, you default to one of the appropriate substitutes, whichever is best, and then roll the skill you defaulted to; you apply only 1/3 the default's bonus, and ignore the ability modifier if positive; the attack's ability modifier is used if negative, not the default-skill's ability modifier. Defaulting also incurs the usual -4 non-proficiency penalty.
Eventually I may post the full document of these variant D20/3E D&D rules to the House Rules forum or something, if I ever get around to finishing 'em. {:^D
Attacks:
When an attack is made, the attack roll is simply a skill check with the appropriate weapon skill, or an appropriate default skill if necessary, and damage is rolled as per 3E D&D. However, the damage is then multiplied by a size factor of 1 if the wielder is Fine-sized, or 2 if the wielder is Diminutive in size, or by 3 if the wielder is Tiny, by 5 instead if the wielder is Small, or by 8 if the wielder is Medium-sized, by 12 instead if the wielder is Large, or by 20 if the wielder is Huge, or 35 instead if Gargantuan, or multiplied by 50 if the wielder is Colossal in size. This matches the multiplier for Hit Points, such that a creature of equal size with its opponent will deal similar damage, though dependant on the wielder's Strength, prowess, and weapon choice. The damage multiplier is increased, however, by 1 per 5 points by which the attacker rolls above the defender's own roll. Melee weapon skills are normally based off of Strength, but some may utilize Dexterity instead with a proper weapon. Likewise, Projectile and Throwing skills are normally based off of Dexterity, but some may utilize Strength instead with a proper weapon.
An attack hits if the attacker's weapon skill check beats the defender's check, with either the Dodge skill or an appropriate weapon skill. A roll of natural 20 on the attack roll is an automatic hit and threatens a critical hit, as normal, and likewise a roll of natural 1 on the attack roll is an automatic miss as normal. However, a roll of natural 20 with the defender's Dodge or weapon skill results in an automatic miss, even if the attacker had rolled a natural 20. A roll of natural 1 by the defender instead results in an automatic hit, while also leaving the defender's effective Dodge and weapon skill defenses for the rest of the round at a total of 1 plus their normal modifiers. Any Melee, Projectile, or Throwing skill can be used in place of Dodge to defend against an attack that uses the same broad type of weapon skill. For instance, any Melee skill can be used to defend against a melee attack, while any Projectile skill can defend against a ranged projectile attack, while any Throwing skill can defend against a ranged throwing attack. However, weapon skills are often less effective than Dodge when used in this manner, as they impose penalties when used to defend against certain kinds of attack (dependant on the weapon skill defended with). Also, certain kinds of weapon may be capable of defending against more kinds of attack than normal. For instance, Melee (Staves) can be used to defend against Throwing (Spears), and certain feats may allow more such exceptions to the standard defense rules.
Armors, shields, and other items or effects may reduce the damage of an attack, by providing an Armor Class bonus. AC directly reduces the damage suffered by an attack, but most sources of AC can be circumvented by a skilled attacker if they use certain weapons, tactics, or techniques. Shields also improve the effectiveness of the Dodge skill, though armor has the capacity to impede the Dodge skill at times.
When armors, shields, or weapons are used to defend against an attack, or merely to block damage through the use of Armor Class, they usually suffer at least minor damage from the impact, depending on the force and type of attack. Only very powerful attacks will cause significant damage to defending items, when not specifically directed at damaging the items instead of the defender (directly attacking an opponent's weapon, shield, armor, or other item on their person is considered a sunder attempt).
Experience Point Purchases:
XP Cost Benefit
5 +1 rank in a skill, for which improvement methods are currently easily accessed
10 +1 rank in a skill, for which current opportunities for improvement are challenging
15 +1 rank in a skill, for which improvement is very difficult to achieve at present
5 Advance a weapon skill's proficiency one degree with a closely-related weapon*
10 Advance a weapon skill's proficiency one degree with a distantly-related weapon*
*Based on any weapon of that skill that the character already has full proficiency with
Applying Strength - .......Specifically, a character's Strength modifier directly applies to the following: Melee skill checks; damage with melee attacks, throwing attacks, grapple attacks, and certain projectile attacks (as noted in individual projectile weapon descriptions); ........
.............In order to wield a weapon, even an improvised weapon, without suffering circumstance penalties for weapon weight, a character must meet at least a certain requirement in Strength. Wielding a weapon too heavy for such effective use incurs certain penalties. This Strength requirement is a score of at least 2 per 1 pound of the weapon's weight, rounded down. The Strength requirement is increased by 50% if wielding the weapon one-handed. However, it is increased by a separate 50% for characters of Tiny-size or smaller (stacking to a 100% increase if they wield the weapon one-handed). This Strength requirement is 2 for characters of Tiny-size or smaller trying to wield a weapon one-handed, if the weapon weighs at least half a pound but less than a full pound. Reduce the Strength requirement separately by 50% for characters of Huge-size or larger (stacking to a 0% total adjustment if trying to wield the weapon one-handed).
Applying Dexterity - .......Specifically, a character's Dexterity modifier directly applies to the following: Projectile skill checks; Throwing skill checks; .........
Skills: (temporary list)
Key Ability Name Defaults (halve their bonus when used in its stead)
Strength Melee (Axes) Melee (any Clubs, Polearms, or Swords)
Melee (Clubs) Melee (any Axes, Polearms, or Staves)
Melee (Flails) Melee (any Grappling or Staves), Throwing (any Slings)
Melee (Grappling) Melee (any Flails or Unarmed)
Melee (Knives) Melee (any Swords or Unarmed)
Melee (Polearms) Melee (any Axes, Clubs, or Staves)
Melee (Staves) Melee (any Clubs, Flails, or Polearms)
Melee (Swords) Melee (any Axes, Clubs, or Knives)
Melee (Unarmed) Melee (any Grappling or Knives)
Dexterity Dodge Tumble
Projectile (Arquebus) Projectile (any Crossbows or Firearms)
Projectile (Artillery) Projectile (any Firearms)
Projectile (Bows) Projectile (any Crossbows), Throwing (any Shotputs)
Projectile (Crossbows) Projectile (any Arquebus or Bows or Siege)
Projectile (Firearms) Projectile (any Arquebus or Artillery or Crossbows)
Projectile (Siege) Projectile (any Artillery or Crossbows)
Throwing (Darts) Melee (any Knives), Throwing (any Shotputs or Spears)
Throwing (Shotputs) Melee (any Clubs), Throwing (any Slings)
Throwing (Slings) Melee (any Flails), Throwing (any Shotputs)
Throwing (Spears) Melee (any Polearms), Throwing (any Darts)
Note, maximum skill rank should probably be equal to 5 + character level, possibly allowing access to a skill focus sort of feat that increases the max rank possible by 2 to 4.
Feats:
Name Prerequisites
Armor Proficiency (Heavy) Armor Proficiency (Light), Armor Proficiency (Medium)
Armor Proficiency (Light) None
Armor Proficiency (Medium) Armor Proficiency (Light)
Combination Weapon Proficiency Melee or Projectile or Throwing (all that apply) 4+ ranks
Shield Proficiency (Light) None
Shield Proficiency (Heavy) Shield Proficiency (Light)
Note, light shields include bucklers, tessens, war fans, war cesti, gauntlets, and small shields, when used for defense, while heavy shields include large shields, tower shields, kappa shells, and similar barriers
