A couple of options immediately spring to mind:
- Aikido
- the internal Chinese arts of Hsing - I and, Ba Gua (Pa Kua) and Tai Chi
Aikido is reasonably well known and needs no introduction. The internal arts though are relatively unknown to a lot of people. They are separate arts and they are quite different to your tradition karate/TKD/jiu jutsu, they do have some similarities with kung fu. However I think you will be amazed at how they complement and enhance your current styles.
This is a brief explanation of how the external and internal arts differ (excerpted from the first link below, which appears to have been first published in the magazine Inside Kung Fu):
Although body mechanics and movements of external martial arts may vary greatly from style to style, the major difference between these and the internal styles is that external styles, while generating power through the coordination of the body as a whole, lack unity of motion in the internal arts sense. For example, many external martial arts strike using the power of the waist and upper body from the base of a stable stance, the blow would be relaxed during delivery, then tightened for an instant at impact This type of strike is capable of generating a great amount of power, with the force being produced mainly by the waist and striking limb. This whipping of a limb and tensing at impact is referred to as "sectional power" and differs from the whole body power of internal martial arts. The sequence of training in external martial arts also differs in purpose. In the early stages of training, external martial arts place greater emphasis on increasing strength and endurance as the "raw material" to be refined later into precise technique. Whereas the goal of internal style stance training is to train the nervous system into the feeling of a unified body, the external martial artist stands to increase the strength, endurance and flexibility. As a consequence, external stance training is usually lower and wider than that of the internal. Although an oversimplification, it may be said that the internal martial artist stands to cultivate feeling, while the external martial artist stands to develop strength. External martial artists often spend considerable time conditioning specific areas of the body, either to withstand impact or to increase sectional power. An external martial artist may especially condition the head, fists, elbows, shoulders, fingers, or emphasize a specific movement, resulting in the development of a specialized weapon. This is another example of the development of sectional power in the external martial arts. Once the martial artist has a strong foundation, form and technique training begins. Once again, the forms and techniques emphasized in external styles are designed around the sectional power developed through basic training.
In summary the moves that comprise the internal arts are executed differently than the external styles; they don't just rely on a measure of speed or power to determine effectiveness. The internal arts also tend to concentrate on attacking the internal meridians of the opponent, whereas the external arts concentrate on external vital points and the external meridians. What this means for you is less hard impact on your body, and a whole bunch of new techniques that will both complement and enhance your existing techniques.
Some references I dug up quickly (I'll endeavour to find some better links in a few hours time):
