Note: The following headcanon is derived from the 5e version of the Great Wheel, which has neither quasi-elemental planes nor Hinterlands.

The Outlands has elemental gate-towns, eight in number, organized in a circle midway between the Spire and the outer gate-towns. Even closer to the Spire is a triangle of gate-towns leading to the Material, the Feywild and the Shadowfell.

The "gates" the towns are built around are no mere portals. They are actually bigger on the inside, each containing an entire plane.

Two suns orbit the Outlands, one made of positive energy and the other of negative energy, bringing day and night. Should you fall from the edge of the Outlands, you would plummet either toward one of the suns, or past them and into the Far Realm.

In short, the Outlands is the Great Wheel.