Create six new semi-elemental planes that each corresponds to a pair of Elemental Planes. These don’t have to behave exactly like the combination of the two elements (e.g. the Plane of Water and Plane of Earth intersection doesn’t have to be a Plane of Mud); the inspiration for them is levels like SpliceHack’s additional semi-elemental planes.

Randomize the order of the elemental planes, and insert the appropriate three semi-elemental planes between each pair of elemental planes. This leads to an End Game that is highly different from game to game, but has continuous transitions at the same time.