Already out in KMS. This potential is beyond unique and its called Legendary (green colour). It adds many cool stuff like 12% main stat, % extra mesos/drop rate. More %boss, new decent skills such as decent SI, decent adv bless and decent CO.
You need master cubes to get from unique to legendary. These cubes are more expensive (im guessing 3k in gms), and the rewards you can get from master cube's fragments are better, such as AEE and special potential scrolls.