I've been working on a pen and paper RPG system based on the d20 system. Here's a few tidbits.
-Weapons and weapon properties: Weapons will often have intrinsic properties. These properties often give some unique minor bonus, or even a penalty associated with them.
1. Noisy: Using this weapon gives enemy characters an immediate Perception check to notice your presence (with a +10 bonus). This property can be negated by equipping a "Supressor" upgrade. Example weapons with this property: Any form of gun or explosive weapon, chain saw.
2. Finessable: You may use your character's dex bonus for melee attack rolls instead of str. Example weapons with this property: Unarmed, katana, dagger, rapier.
3. Armor Piercing: Ignores 3 points of damage reduction on characters hit by this weapon. Ranged weapon ammunition with this property takes a -2 penalty to damage rolls. Example weapons with this property: Scythe (piercing mode), armor piercing ammunition (guns, bows), bill hook (yes, the signature weapon from Higurashi is being referenced here).
4. Psychopathic: Upon the first round this weapon is used, all enemy characters within 15 feet of the person who used it must make a will save. Those who fail are shaken for 1d3 rounds. Example weapons with this property: Bill hook, chain saw, scourge, meat hook.
--Weapon proficiency is based on groupings. Example weapon groups are swords, guns, bows, cross bows, axes, hammers, spears, unarmed, and maces. More exotic weapons (like katanas and bastard swords) just require the Advanced Weapon (Category X) to be proficient in.
-Damage attributes include the following: Slashing, piercing, bludgeon, fire, ice, water, lighting, earth, wind, holy, unholy, ordered, chaotic, explosive (generally effective against slimes and constructs), corrosive, sonic, and energy. For simplicity, monsters are categorized as strong (half damage), weak (double damage), or immune (take 0 damage). For attacks with multiple attributes, calculate damage due to weaknesses and resistances seperately for each attribute.
-Defense: Defense (to prevent being hit) is based on the sum of 10+ class modifier (which does level up with the character) + appropiate stats (dex or depending on character wis) + shield bonus. Fighting from high ground gives a +2 bonus to attack rolls. Being in cover gives a bonus to defense depending on the grade of cover. Armor (except shields) instead confers damage reduction.
-Degrees of success and failure: This mechanic is designed to make good and bad luck less lethal to characters. Every 5 points above and below the required roll to succeed is a degree of success or failure. Natural 20's confer an automatic success to the attack roll, saving throw, skill check, etc, but you still add total modifiers to determine the degrees of success (rolling a 20 still confers 1 degree of success even if the total modifiers were below the target roll). Critical hits require two or more degrees of sucess on a "threat" roll (after modifiers). A natural one always fails, but modifers are added to determine degrees of failure (a natural one that would have suceeded instead gives 1 degree of failure).
One degree of failure usually has no bad stuff happening other than just not suceeding at the task (no net progress for skill checks). 1 degree of failure on a saving throw confers a lesser effect depending on the technique or spell. 2 degrees of failure has bad stuff happen on skill checks and the normal effect on a failed save.