Man, did we have bar brawls when we played D&D in junior high and high school. I think some days our D&D campaign would have been better titled "Punch a Cavalier in his Arrogant Face: The Role Playing Game."
So your party is in yet another bar brawl. When you roll to hit, also roll an extra 1d6. If you hit, refer to the following table for the results:
1 One-Two Punch (you are +1 to hit this target next round)
2 Kidney Punch (victim is -1 to hit next round due to pain)
3 Low Blow (if male, victim must save vs paralysis or lose next action)
4 Uppercut (+2 damage)
5 Head Butt (victim must save vs paralysis or be stunned next round: may move or attack but not both)
6 Haymaker (+1d3 damage)
This is loosely inspired by the brawling and wrestling tables from AD&D 2nd edition, and should be usable with your favorite version of classic D&D. I went with a d6 to keep the number of possibilities small, thus better creating a coherent sense of a particular style of fighting. This way I can do other styles like so:
Halfling Folkstyle Wrestling
1 Foot Stomp (victim may not move next round)
2 Double Leg Tackle (victim must save vs Paralysis or be knocked down, automatically losing initiative next round)
3 Bear Hug (victim must save vs Paralysis or be held next round; fighter may apply damage to held victim next round without an attack roll)
4 Clothesline (victim must save vs Paralysis or lose next action getting up from the ground)
5 Ear Clap (victim is -1 AC next round from stun)
6 Uppercut (if victim is halfling sized, +2 damage; if victim is human sized, treat as Low Blow)
Monkey Kung Fu
1 Drunken Monkey (fighter gains +1 AC for one round)
2 Stone Monkey (fighter takes -1d3 damage from all attacks for one round)
3 Lost Monkey (fighter is at +1 to hit due to the trickiness of their attacks)
4 Standing Monkey (fighter attacks from distance; victim must move in again to make a melee attack)
5 Wooden Monkey (+1d3 damage from barrage of attacks)
6 Monkey Steals the Peach (if male, victim must save vs paralysis or lose next action)
Or weapon-based systems like so:
Sword and Board
1 Defensive Slash (+1 AC)
2 Hold the Line (opponents may not move past the fighter)
3 Shield Bash (+1d3 damage)
4 Shield Rush (victim is knocked back one square and fighter may enter the breach)
5 Shield Pin (victim must save vs. Paralysis or be pinned and unable to move next round)
6 All Out Attack (+2 damage)
Or monsters like so:
1 Rending Bite (victim is -1 to attack next round from pain)
2 Piercing Bite (victim is -1 AC next round from damage to armor)
3 Shaking Bite (victim is shaken up and takes 1d6 additional damage)
4 Chew 'em Up (save vs paralysis or be caught in dragon's jaws; dragon may deal bite damage next round without rolling to hit)
5 Spit 'em Out (roll dex or lower on 1d20 or be spit out of melee range; must spend next round getting back into range or use a ranged weapon)
6 Flaming Bite (+Dragon's HD in additional damage from breath weapon crackling out during the bite)
The effects assume you're not using minis, so they abstract things a bit. If you're using minis, you can probably translate the effects of throws and the like into inches or squares pretty readily. It would be fairly easy to expand this into a bunch of different fighting styles and weapon styles. Fighters could learn one unarmed or weapon style for every so many levels (e.g., 1st, 3rd, 6th, 9th). Rogues could learn a style or two (e.g. one at 3rd level, another at 9th). Clerics could learn one at 3rd and 9th as well. Magic Users might be allowed one at 6th, if at all. Monks/Mystics might get as many as one style every level, with the requirement that 2/3rds or more of the styles be unarmed. Anyone with more than one style must declare which style they're using at the beginning of the round, before dice are rolled. If you don't declare a style, it's assumed that you are using the style you used last round. If it's the first round of the fight and you don't declare a style, it's assumed you're just attacking normally.
The effects are kept small in most cases. I don't want these to circumvent the basic "to hit roll + damage roll vs hit points" economy of the game, just add a little random, fun spice without requiring players to improvise elaborate descriptions of what they are doing each round.
- No effects last more than one round, to avoid adding a bunch of modifiers to track.
- Only one style can be declared active per round.
- The really minor effects just happen.
- More serious effects (leading to lost actions, etc.) provide a save to avoid the effect.
The system is pretty loose, so GMs will probably have to adjudicate details and extrapolate from context (e.g., A halfling can't take down a dragon. Should be able to tackle a human to the ground, though, if you're running a somewhat unrealistic adventure style game.)
Also, here's another take on kung fu for D&D, in case you're in the market and it better fits your style of game.