Transitive Planes

There are planes that are featureless, endless realms that exist primarily to be methods of travel between planar levels. They are connected to Eden, the Material Plane, and are the methods of travel used for those who want to leave or enter.
The Ethereal Plane is a misty swirl of muted colors that sits between the Prime Planes and the Inner Planes. Because the Ethereal Plane directly overlaps with Eden, allowing ghosts and spirits to exist on both planes at once, it was once considered merely a "child" plane to Eden rather than its own separate entity. Because it must be crossed to access the Inner Planes, however, scholars decided that it should be classified as its own plane.
The area of the Ethereal Plane that directly overlaps with Eden is known as the Border Ethereal. It is where spirits dwell, and where spells such as etherealness and true seeing allow one's mind or body to travel. Beyond that is the swirling sea of the Ethereal Plane, through which you must travel to reach the Deep Ethereal, which allows you entry into the Inner Planes. The Deep Ethereal is also where "pocket dimensions," often created by spells and Bags of Holding, are incubated.
When you must travel the Ethereal Plane, you must walk to your destination. While you walk, you must keep your destination clear in your mind, and not your thoughts falter for even a moment. If you do, you will go astray and become lost in the Ethereal. Once you are lost, there is little hope of you finding your way back. You may find yourself in the correct Plane but the wrong location, or the right location in the Wrong Plane. Or, worst of all, you may never find your way out.
There are creatures in the Ethereal Plane, other poor souls who lost their way and now have only the desire that others share their fate. These Wanderers may meet you on your way, and will attempt to tempt you into coming with them on a "shortcut." Some truly do wish to save you from their brethren, but most want only to claim your soul for the Ethereal.
The Astral Plane, known more commonly as the Astral Sea, is the realm of thought and dream. Travelers roam as disembodied souls towards the Outer Planes to reach their afterlife. It is the only way to reach the Outer Planes from the Prime Planes. It is a great and beautiful sea, black and silver with distant twinkling lights like stars. Erratic whirlpools of color flick in midair like spinning coins, enticing wandering souls. Solid matter is rare here, but occasionally you will find the carcass of something that could not, or would not, complete its final journey.
"Ferries" are the fastest mode of travel across the Astral Sea. Some of them are literal ships of starstuff, steered by creatures beyond the comprehension of celestial or mortal. They are the only creatures capable of surviving extended periods of time in the Astral Sea. Many times, you will have to find a random ferryman, but if you happen to know the true name of a ferryman (which they do not give freely), you may speak it aloud when casting your spell to travel, and you will find yourself in their path.
Every ferryman requests a different currency, which can include favors, memories, beets, or anything else you can conceive. Once you pay them, they will take you across the Sea, often making frequent stops to pick up or drop off other passengers on their way to their afterlife. Ferryman that you are closer to, or with which you have a frequent working relationship, may let you keep a running "tab" that you are expected to pay before you die. If you fail to pay a ferryman, your soul is immediately forfeit and you are bound to the ferryman's vessel for eternity. For those that need no crew, this means being strung up and dragged behind the vessel as it travels.