The normal description of the Astral plane is a silvery big nothing where characters can enjoy Superman fantasies (i.e. they can fly without wings). Yes, there is the occasional divine corpse and a few planetoids populated by gith, but that is still pretty boring. Here are some ideas for variants:

1. A forest where the portals are small pools of water. Yes, that is a rip off of Narnia, but that is still an excellent idea and visual. One could populate the forest will all kinds of monsters looking for travelers. I would look to fairy tales to populate mine.

2. An infinite labyrinth where the portals are doors or arches. Minotaurs are the obvious choice to populate it with, but I think a terrestrial form of morkoth would make for a horrid surprise.

3. An ocean where the portals are rings of algae, coral or bubbles. Those projecting into this Astral variant wouldn’t need to worry about breathing, but those people Plane Shifting or using Gate would. Instead of tanks, gills should work and last much longer than Water Breathing.

4. A massive molecule where the portals are glowing spheres or circles. No, I didn’t get the idea from The Atom, but rather a much older source- The Girl in the Golden Atom. All kinds of weirdness can be found in this subatomic world, much more than Ray Palmer ever found.

5. Ruins of the past in an elemental dominated desert where the portals are fragile arches. Every time a civilization falls, it slowly slips into the Astral plane sort of like the fate of Sarnath. Usually this is just buildings, but occasionally monsters that thrive in deserts can be found as well. The air, fire and earth elementals were set as guardians to watch over the ruins and prevent looting- the gods use the plane as a museum and don’t want the past damaged.

6. Infinite rooms and hallways where the portals are doors. The walls are very tough, but some monsters have been able to chew through them, making gaps and destroying some of the portals. Unlike many of the other variants, this one may have a large population of humanoids. Markets of all sorts line larger hallways and fill rooms that anchor specific communities. Other rooms contain libraries with books from across the multiverse. The librarians tend to be very protective, and powerful, so few try to steal from them. And then there are the art galleries. People (of all shapes and sizes) come to display, see and purchase works that are alien to them. Mortals (those from the material plane) tend to buy a lot of elemental and outer planes art.

7. A glacial field where the portals are small pools of water. The ice contains untold numbers of frozen people, monsters, technology and magical objects. Defrosting them is difficult and requires a Wish or equal magic. It is thought that this was a prison but strangely one can find people and technology from the future as well as the past. Sages usually throw their hands up and try not to think about it.

8. Massive clouds of Good, Evil, Order and Chaos where the portals are glowing spheres or circles. The clouds are dangerous even to those with the same alignments and the lightning that strikes between them can be fatal. Those who survive the lightning are usually warped in mind and body, the result of feedback between the original and target energies. There is no war here, just natural processes that mortals should avoid at all costs.

9. The Great Machine has portals in the form of gaps in its structure. It runs the multiverse and its core is indestructible. Those wanting to make changes can do so by adding new machines connected to the old. These new parts are just as vulnerable as the materials that make them up. This is a place of war where giant armies of celestials, fiends and everyone else that cares exist to protect their own creations and destroy their foes’. Traveling this Astral Plane is only for the most foolhearty or powerful. The machine does not have to look like an engine. It could be made of flesh, crystal/glass, solid time (whatever that looks like), telepathic waves, soul fragments or what have you.