Planescape is the sandboxiest of the D&D settings; though the task of the GM creating new locales was somewhat addressed in the published material, such was really just a rough skeleton. What Planescape needed was a robust toolkit for creating new planes/worlds. Game designer Kevin Crawford, whose fans regard as a master of sandbox game design, has finally supplied that missing world-building toolkit with his upcoming Worlds Without Number system book. The default gameplay mechanics of Worlds Without Number are Crawford's elegant refinements of OSR d20, but his world-building tools are system-agnostic and could be used with any RPG game [such as Planescape]...
https://www.kickstarter.com/projects/1637945166/worlds-without-number