I always thought a "Yin and Yang" serious of balls would have been cool. Yin is black, and represents darkness, night, the moon, and passiveness. Yang is white, and represents light, day, the sun, and aggressiveness.
Yang being a White Pearl, Yin a dull Black solid, then a 3rd, White/Black Hybrid. I had this idea a few years ago. Since then, the 400A has been released. Which is what I pictured for the Yang, but stronger. The Yin would be something similar to the Bad Intentions or Guru. The Hybrid could be called, Yin Yang and look similar to the Punch Out by Storm. I envisioned stronger symmetric cores for these, like the Centripetal from Storm with the R2X or R2S covers.