Superior Two Weapon Fighting

It is still unclear whether if this is going to be introduced as a feat (especially for non-rangers), and in that case what prereq is going to be.

TWF feat prereqs;
 * Two Weapon Fighting: Dex 15 (free for Ranger 2)
 * Improved Two Weapon Fighting: Dex 17, BAB 6 (free for Ranger 6)
 * Greater Two Weapon Fighting: Dex 17, BAB11 (free for Ranger 12)
 * Superior Two Weapon Fighting: ???