If you are looking to get "creature spirits" involved... here's a method for that...
The Astral plane is the place where all souls go when they die. ALL SOULS. Every planet in the universe and every religion on each of those planets each have their own "pocket" of the Astral plane where their souls go, based upon their own beliefs. So for Earth, Christianity's "Heaven", "Hell" and "Limbo" are located in their own spaces on the Astral plane... as well as Earth's Nirvana, Valhalla, Hades, Hel, etc. etc. On top of that... other inhabited planets in the universe (planets whom Earth has had no contact with thus far in reality) ALSO send their dead spirits to the Astral Plane in their own individual pockets.
Now normally each individual pocket of the Astral does not and cannot connect with any other pocket (so a spirit in Heaven couldn't move to Valhalla for example). And by the same token, no spirit from Earth would ever meet a spirit from any other planet in the universe (which might very well be filled with all manner of creature or monster).
However, if perhaps the Astral plane has begun to rip just a bit... and pockets of the Astral have begun bleeding over into each other... you might just have an invasion of one of Earth's realms of afterlife from places where you REALLY don't want meet their spirited inhabitants.
Your Astralnauts could be a living team that the spirits of Earth bring across to the Astral plane to battle the invading forces from any of the other planet's realms of the dead. It's only BECAUSE they are living that they are able to affect changes in Earth's realms of the dead, because the dead themselves cannot do it. We know this because of all manner of myth where spirits stuck in Hell, Hel, and Hades were always unable to get out of whatever predicaments they were put in for eternity. And spirits in Heaven can't affect it because their afterlife is made so perfect that they haven't had ANY NEED to try and affect it.
This scenario gives you opportunity to mix a lot of Greek, Norse, & Egyptian mythology in and around a Frighteners/Ghostbusters/Final Fantasy spirit fighting squad. You can choose which Earth spirit realms might have rips to start with (so perhaps it is Heaven that is the first to get invaded, but slowly other realms also get broken into... leading to interesting stories where the heros team up with dead heros from Earth history, as well as dead heros from Greek myth, Native American myth, Japanese myth, etc. etc.
Plus you can then get some really terrific plots going when it is discovered that Hell has been ripped into, and perhaps some evil spirits from Earth's history have begun teaming up and/or leading the invasions from the other realms. And if the Astralnauts can't fix things, eventually the barriers between ALL the realms of the dead from all planets in the universe might break down... and you'll have a huge donnybrook for all dead people for all eternity.
The Astral plane is the place where all souls go when they die. ALL SOULS. Every planet in the universe and every religion on each of those planets each have their own "pocket" of the Astral plane where their souls go, based upon their own beliefs. So for Earth, Christianity's "Heaven", "Hell" and "Limbo" are located in their own spaces on the Astral plane... as well as Earth's Nirvana, Valhalla, Hades, Hel, etc. etc. On top of that... other inhabited planets in the universe (planets whom Earth has had no contact with thus far in reality) ALSO send their dead spirits to the Astral Plane in their own individual pockets.
Now normally each individual pocket of the Astral does not and cannot connect with any other pocket (so a spirit in Heaven couldn't move to Valhalla for example). And by the same token, no spirit from Earth would ever meet a spirit from any other planet in the universe (which might very well be filled with all manner of creature or monster).
However, if perhaps the Astral plane has begun to rip just a bit... and pockets of the Astral have begun bleeding over into each other... you might just have an invasion of one of Earth's realms of afterlife from places where you REALLY don't want meet their spirited inhabitants.
Your Astralnauts could be a living team that the spirits of Earth bring across to the Astral plane to battle the invading forces from any of the other planet's realms of the dead. It's only BECAUSE they are living that they are able to affect changes in Earth's realms of the dead, because the dead themselves cannot do it. We know this because of all manner of myth where spirits stuck in Hell, Hel, and Hades were always unable to get out of whatever predicaments they were put in for eternity. And spirits in Heaven can't affect it because their afterlife is made so perfect that they haven't had ANY NEED to try and affect it.
This scenario gives you opportunity to mix a lot of Greek, Norse, & Egyptian mythology in and around a Frighteners/Ghostbusters/Final Fantasy spirit fighting squad. You can choose which Earth spirit realms might have rips to start with (so perhaps it is Heaven that is the first to get invaded, but slowly other realms also get broken into... leading to interesting stories where the heros team up with dead heros from Earth history, as well as dead heros from Greek myth, Native American myth, Japanese myth, etc. etc.
Plus you can then get some really terrific plots going when it is discovered that Hell has been ripped into, and perhaps some evil spirits from Earth's history have begun teaming up and/or leading the invasions from the other realms. And if the Astralnauts can't fix things, eventually the barriers between ALL the realms of the dead from all planets in the universe might break down... and you'll have a huge donnybrook for all dead people for all eternity.