So anyway I created these simple but relatively abstracted martial arts systems. Any one school should give a practitioner a slightly different feel in a fight and a dedicated PC who went around and put in the effort to learn more than one style should find themselves with a series of useful but not cumbersome or redundant abilities.
"Should"--these are untested. Feel free to test them--#1 seems to me, at the moment, the best-designed.
Consider these martial arts to be kind of like magic items--you have to adventure to find them but, once found, yeah, you're better than you used to be.
In order to learn a school of martial arts you have to:
A) Find a teacher of that school--which should be as hard as finding a magic item.
B) Spend half the xp that would be necessary to go from the bottom of the level you're at to the bottom of the next level.
If an opponent makes an attack in close combat and misses, the opponent takes damage from your automatic skillful counterstrike as if the foe had successfully struck himself/herself. This ability counts as your attack action for the round (or the next, if you've already acted), does not have to be announced in advance and can be used once per opponent in any given encounter.
(Many D&Ds give the option to "fight defensively"--not striking a blow, but improving AC by 4. This martial arts style may be used in conjunction with this combat move.)
2. Thousand Monkey Style
Look at your Wis, Dex, Str, Con and level. Take the highest one.
First, you may also treat this score as your ascending armor class when not wearing armor, or use its modifier if wearing only leather.
Second, once per day, if you fail an ability check or to-hit roll that you would have made if your score had been equal to this higher ability score, you instead succeed. (This is how wizened old men manage to kick everyone's ass in kung fu movies--they are using their level for checks.)
This style may be "relearned" multiple times to gain the ability to substitute scores more than once per day, up to a maximum of three times.
3. Northern Mountain Fist
Your damage dice explode. (This is why kung fu people use crappy weapons like throwing stars or their fists or random pieces of wood--a d4 is way more likely to explode than a d8.) If you roll max damage twice in a row when exploding and the damage die is at least d4, you add your whole level to the damage.
Also, when unarmed, your initiative improves. It is +1 if the game usually does not have initiative modifiers standard and +1d4 each round if it does.
4. Southern Island Styles
You know a series of unarmed combat moves which make your strike in combat able to act in most practical ways identically to a single melee weapon of your choice--reach, damage, space needed, etc. So, for example, if your style gave you the abilities of a chain, you'd be able to easily entangle limbs at 10'. These moves have names unrelated to the weapons they imitate so, for example, a scimitar-imitating style might be called the Raging Sting Talon.
(Bruce Lee's 1-inch punch is clearly a style that imitates a dagger--doing d4+str damage at only the distance one would need to to use a knife. A full-on flying kick would be more like a 2 handed sword--big damage, but needing a lot of room.)
You also have the option to choose a style imitating a throwable melee weapon such as a throwing axe or dagger--this allows the martial artist to leap, traveling the distance the weapon would travel, strike, and land near the target or leap back to where they began (and only there) on their melee turn. (Arrows, spears, javelins and other long-distance projectiles may not be imitated.)
Multiple Southern Island styles may be learned from the same teacher--the PC chooses the weapon in each case.
5. School of the Purple Lotus Emperor
Once per day, the character may focus ki energy and roll a d30 in lieu of whatever die or dice the situation normally calls for. The choice to roll the d30 must be made before any roll. The d30 cannot be rolled for generating character statistics or hit points.
6. School of the Golden Mantis
Gain a Kung Fu number. "Re-learning" this style of martial arts through more practice (spending more xp) can grant a PC a maximum of 5 kung fu numbers.