Well we run the Astral Plane as the Psychic's domain. Mages can get their with the funny little tricks and and spells, but they are outgunned and really shouldn't bother. Unless they have a psychic to keep them safe.
Everything touches the astral plane. Well, most everthing touches the Astral Plane. There are always exceptions.
What happens to a devine realm or a hell when the gods and demons there no longer hold sway in the mortal mind? People forget, stop worshiping, stop fearing, or die off. That PPE is no longer pumped into the belife system that held these demons and gods in power. They float off into the astral plane to fester and rot.
I've seen **** that would turn you white! - Winston Zeddemore, Ghostbusters.
But not just the realms of heaven and hell are out there. So to are the gods and demons. No longer worshiped, no longer remembered. Shadows of their former selves. Languishing in a forever sea of nothing or sitting atop thrones in places no one has spoken about in millenia. Nothing. Second after second of nothing. Decade apon decade of nothing. The mind goes. Becomes trapped by the maddening nothing and fading memories. These things are out there, and are the reason why you don't go into the deep astral plane without good cause and some serious power.
Getting there. How does a psychic get into the plane, physically?
That kind of depends on your GM. There aren't any rules to my knowledge beyond the standard making mages or random rifts do it for you. To facilitate our magic hatin' psionic dominating play style, we have the astral gate. It's a master/super psionic ability, cost 100 ISP to activate, requires 1d4x10 minutes of meditation and a large reflective surface such as a pool or mirror to act as the gate. It must be big enough for whatever it is one wants to send through to the plane. IE, a person requires a man sized mirror, while a jet fighter or really big demon lord would require a really big mirror, like the side of a mirrored building. Telepathy can be used with this allowing the character with Astral Gate to tap other psychics (a character with ISP AND a psionic power) to contribute ISP to the opening, and all invovled must meditate (and hold open/pay maintainance on) telepathy. Characters with the mirror walk ability but are away from NB Earth/the night lands, who use their mirror walk will end up on the plane instead.
Astral projection when one is one the astral plane will either allow the psychic to explore the plane as if it were the real world (someone on earth using AP to explore the same physcial realm only now that one is on the plane, it is their base state) or can project into some other plane (like where they came from) as if projecting to the astral plane. Use the same rules, just flip them around for point of origin.
Leaving the astral plane works the same. You just need to find a mirror on the target plane you want to enter, then spend the points and meditate. And hope nothing really really nasty comes by while your defenses are limited...
I said that mages don't belong here. There is a reason for that. Psychics can manifest their powers in a varity of ways. Summon stuff up, like astral golems, change the sceenery to fit their moods. That includeds changing the laws that govern magic.
A psychic manifesting their will over the plane has to decide on the law that varies from the plane, change it (burns one action doing so) and has to pay one action each melee they wish to keep the effect going. Really powerful effects might cost more actions, depending on the GM's ruling. One can't cancel the laws governing psionics here or any other ISP based power. Range of the effect is the psychic's ME x10 feet+10 feet per level of experience Breaking the law can be done, if the law breaker makes a saving throw against psionics. Machines without wills of their own (like a car or gun) in violation of the law will fail to function but be unharmed. They just don't work here. Machines with wills of their own, due to possesion or mental transferance, would use the relivant psi-save of the controling entity (Skelebots would get a save but a Triax mind transferance battle bot would). The second the psychic stops thinking about (because of death or knock out darts in the ass or what have you) or just lets the effect lapse, the new laws end and astral plane immedately reasserts itself. No, you can't do it in someone's astral domain.
Psychics trying to compete with each other over a control of a law roll a d20 +Character level+ psi-saving throw modifier, highest number wins. A new check can be made each melee.
All other astral rules hold as normal. The above is my effort to make what we saw in psy-scape, the city, fit with other sources and Between the Shadows.