So I've been for a long time wanted to create an RPG where the characters where hybrids of mortal and spirit, but have gone through many different ideas for settings.  I've finally come up with one I'm really liking so far, though its just a very basic idea right now.  However I want to know people's general thoughts on the idea, and if it is too close to something that has all ready been done, and if so maybe some ideas on how I could change it to be different enough.  So here's some of my basic ideas.

Pantheonverse
Demigods are the primary heroes and villains in Pantheonverse, struggling to understand their purpose and what they are, coping with having both human frailties, and inhuman powers.

Demigods

Demigods are immortal spirits born into mortal form, creating a being that is a hybrid of flesh and blood creature, and spirit.  Spirits are capable of possessing living creatures, but such a being must cope with the soul of the mortal creature, which will in most cases be constantly be warring against the spiritual parasite, or they can expel the mortal soul in which case they do not retain the mortal’s memories, or any of their skills.  A mortal body also is ill-suited to cope with the sudden entry of an inhuman spiritual presence and risks being burned out by it. 
Demigods which are born into mortal flesh however are truly bonded with their mortal coil, and they have no mortal soul to compete with.  Demigods may grow up feeling “different” but are typically unaware of their inhuman nature until metamorphosis.  Their bodies will not be burned out by their spiritual presence as they grow in power, as it was shaped by their spiritual presence since it first started forming in the womb.
Without a physical body, spirits can only interact with physical worlds in a very limited manner.


The Pantheonverse
Pantheonverse is made up of many interconnected realities, each created by a different pantheon of gods, and spirits (for example there is a reality that was created by the Greek gods, and Greek myth is literal truth in this reality).  Each reality is made up of a physical realm, as well as spiritual realms.  One reality connects all these realities, the Nexus.  The Nexus is very similar to our own, it shares our geography, and our history, however magic, spirits, and mythical beasts are very real even on the Nexus, all though their powers are weakened while on the Nexus.  Unlike the other realities, the Nexus was not created by any single pantheon, indeed the nature its creation is a mystery.  On the Nexus all though their powers are less than else where spirits of many pantheons interact more frequently than else where.  Most mortals on the Nexus are unaware of the supernatural beings in their midst, but in the other realities mortals are very much aware of the supernatural, and the magical holds more power.

Trans-reality portals

Trans-reality portals allow beings to pass from one reality to another. Trans-reality portals may occur on seemingly their own, or be created by magic.  It is said that when the Panethonverse was new naturally occurring portals were common and stable, but now they are rare, and often random.  Portals take many forms, appearing as visual distortions, worm holes, but sometimes mundane objects become portals, and simply walking through a door may mean walking into another reality.

Metamorphosis
A demigod’s metamorphosis is when their inhuman nature finally manifests.  While Demigods from other realities often have a good idea of what’s happening to them, those from the Nexus usually think they are going insane.  During a metamorphosis the demigod will be overwhelmed by visions related to which pantheon they come from, which may very well include memories of their existence before their mortal birth.  They also for the first time will take their Atavistic form, which is a process which may include involuntarily partially shifting several times, before they finally fully transform. 

Revelation and Disbelief on the Nexus
Anyone whom sees a demigod in their Atavistic form on the Nexus may very well disbelieve what they saw, providing some protection from the mortal population discovering the supernatural beings in their midst.  However certainly not all mundane humans will disbelieve, giving rise to those whom pursue discovering more about demigods, and other magical beings, or even forming groups to study or hunt them down.  Fortunately there are usually not enough people on the Nexus whom would take someone that saw a demigod or other magical being seriously to be a threat.

Affects of the Nexus on magic and supernatural abilities
The affects of being on the Nexus on magic are evidence that its creator(s) intended it to be a reality where the supernatural had relatively little influence compared to on other realities.  Still the Nexus has long been a meeting place for beings of many realities.  In the long forgotten ancient past the Nexus was also a battle ground for the powers, but to protect it there are spirits and other beings that will harshly punish any magic being that carelessly uses its powers and reveals itself on the Nexus.  On the Nexus humanity truly holds dominion, and there are higher powers that intend for things to stay that way.

Hello AsuraDemon, and welcome to the Forge!

Do you have a real name we can call you by?

I think that what you've posted here is just fine! However near or similar it could be to other RPGs, I don't really care per se. The ideas sound neat enough to me.

What will change everything for me are the mechanics supporting a certain kind of play experience. I have not seen any indication whatsoever on what that should look like. Is there a GM? Is authoring completely split amongst participants? What's the point of playing your game in the first place? Etc.

You might find it interesting to use the Power 19 as a template to guide you in answering such questions. Or perhaps just the Alt 3 if you prefer a coarser approach first. Check out the other posts of the blog by the way, there's some good stuff about design. The author writes up indexes regularly so as to help navigate his posts.

My name is Megan

well, as far as system, I am using a D20 system but it is not the D20 system.  For character creation players will get a certain number of points to distribute amongst various things, not unlike other point buy systems such as GURPS, and White Wolf's D10 system.  I want players to have a greater degree of ability to customize their characters than a class based system like D&D's D20  allows for (so no classes or leveling up, just buying various things with experience points), but I still will have things within certain limits, I want to really have characters be beings with human limitations right along side with inhuman power. I also reinforce the idea that power both tempts and corrupts, and in order for this to be so I'm working on a way for characters to become more powerful, the more they abuse their power, but also to become more screwed up the more they abuse their power, eventually succumbing to what is called divine madness, which results in them becoming monstrous beings with an alien mindset (horrible demonic beings with once human but now equally alien bodies).  That's another idea I held over from an rpg I was working on and posted about a while ago on this forums, but it didn't end up going anywhere.
So the power corrupts thing is definately important, but I don't want it to be an inevitable thing looming over every character's shoulder either, I want the characters to really have the potential to become true heroes, just as they have the potential to become inhuman villains, this game needs as much or more wonder as horror.

Anyhow I'll also check out those links you listed, can't remember if I've seen them before.

Ok, sounds good, Megan!

Now that we have a rough idea of the setting, rules and the characters, let's approach three other domains that are important to understand your game:

1. What kind of situations will those characters typically be in? Missions? Just living their lives?
Do the characters work in groups or do you rapidly shift from one character's story to the next, in a round-robin fashion?

2. What about the players? You said you wanted them to have a great degree of ability to customize their characters. Why is that? Should characters be really cool/unique/original? Should the players be able to create characters that suit their tactical approaches?

3. What must the GM prepare in advance? Is there any metaplot? What about scenarios/dungeons?

1. Well this is one idea that I also posted on here a while ago, and it was to have the RPG focus on the Hero's Journey.  Integrating the Hero's Journey into game play is still something I'm working on, but I plan on detailing all the key elements and how to integrate them into the game.  That said while characters will probably start out just living their lives, they need to be thrown into an adventure of some sort, I want this to be a game of adventure and exploration, both of the pantheonverse (and that can mean one reality or many), and of the character's own inner self.  I want it to be somewhat like a myth is taking place.  Anyhow that's the idea I'm aming for right now.  Also characters need to have the option of working in groups, or alone.

2. The reason I want a greater degree of customization than D&D and similar games is because I personally have always really liked to feel like I'm coming up with a character sheet that really fits the concept I've come up for my character well.  While with a class, like a D&D wizard for example there's many ways I can put my own spin on a character of that class, but ultimately I'm still going with a character that's seems to me at least very much prepackaged.  Also with systems that don't allow for a large degree of customization you can end up with abilities that make no sense for a character to have, and I prefer to not have that be the case.  This is part of why I sometimes make up my own class for D&D games, but really I prefer to pick and choose what I get as I develope my character more than I can in D&D.  I guess part of what it comes down to is liking to have a greater degree of control over how my character developes.

3. Well the game has a history, it has organizations and individuals that are pursuing their interests, and that might be as close to having a metaplot as I get.  The GM is going to need to play the role of a narrator of sorts, and help deal with rules disputes, and be the player of NPCs, pretty similar to D&D.  The GM should to some extent prepare the adventure, and create the NPCs that the players will likely deal with ahead of time.

I see! I find your aim to focus on the Hero's Journey very interesting.
However, I'm not convinced that, given your design goals for points 1 and 2, the d20 system would be very adapted, but let's tackle some other things before.

Have you got any previous gaming experiences that you could describe (in a thread in Actual Play) that shows how you approached the idea before? Or how perhaps this kind of happened by chance and you then decided to develop further?
This would be really great and has, in my experience, been helpful for lots of people.

What experience do you have of games like The Pool, My Life with Master or Polaris?
All of those are quite interesting in that character creation allows for loads of customization (The Pool is a free game, check it out if you haven't) with impact on how the story develops in-game.
MLwM and Polaris have remarkable mechanisms to structure the narrative in scenes pertinent to the game's goals (respectively horror/psychology/humanity and tragedy/defending causes). Perhaps not quite what you're looking for, but certainly a good place to start, and an excellent read at the least!

well, the only similarities that the system is going to have to D&D style D20 are that this system involves the rolling of a D20, and the adding of modifiers, and that I will probably deal with movement and size in a similar or identical way to D20. 
Magic is going to work completely differently (and it is giving me a hard time), and characters will have virtues and vices, there will be no saving throws or AC (a dodge skill will be used instead).  I'm using a wounds system rather than hit points, and I'm working on a system where characters get different "side effects" from being wounded, such as getting infected, suffering blood loss, or losing an eye, so combat is going to be a lot nastier than in D&D, but hopefully still very epic.

I haven't played a game with the Hero's Journey as a focus, I just think it's an interesting idea, and would like to eventually see how it works.

I have not played any of the games you've mentioned.  I've played D&D, Shadowrun, old World of Darkness games, a few different games friends of mine made up, and a very long time ago Rifts, but I don't remember it well.  I'll look into those games though.  Anyways I think I'm getting some pretty good ideas on how I want to do things in my game, but bouncing ideas off people has been helpfull.

I would suggest giving two articles a read, found here and here.  They go into some details about several other games that have used the D&D base as it's core, and I think it is quite illuminating to the discussion.

That being said, I do like the concept, it reminds me a lot of D&D's Planescape setting.

yeah I like Planescape too, and it is an influence (I guess in a way all the RPGs I've ever played are an influence to some degree), I'm hoping not to make it too similar to planescape however.  My main influences are going to be various mythologies, and folklore rather than RPGs however. 
Also while game play is like Planescape and other D&D games going to be focused on adventuring, one idea I've had that I might use in this game is to have adventuring to be at least in part an exploration of the psyche, and what is going on around the character should at times even become a reflection of the character's inner world, to the point that what's real, and what's imagined should at times become blurred.  When you are a godlike entity sometimes what you imagine and what's going on in your psyche can end up defining what's real anywhow, but since the character is a godlike entity trapped in a puny human body with a puny humann brain you might not exactly have control of when that happens or even understand that it is happening. I don't know how I'm going to accompish this however (though I'm sure using the Hero's Journey will help), I haven't seen any games that really dealt with it (maybe some of the ones you listed do?).

That said after reading part of that article, I think my game's "acts and concerns of roleplaying" will be similar in some ways to those D20 games, but I hope just different enough (especially if I use my above mentioned idea).

I wish there was a way to edit my post, I don't like double posting.  But that said the way my game is most likely to end up a Fantasy Heartbreaker if anything is the rules, since my game is a D20 variant like the games in that article, and I don't see any of the rules I've come up with, D20 or not as particularly original (though I'm really trying to put my own twist on them all). I don't think I could come up with a completely original rules system, at least not without ending up with an unplayable one, and well even then I'll bet it's been all ready done. So yeah my rules design is so far similar in some ways to D&D, it's similar in other ways to a lot of other things. 

The games I mentioned don't follow the Hero's Journey plot, but they do have mechanics that work on the level of scene structure, so you might find them very interesting (check out some Actual Play posts from both games if you want to know some more).
I'm not aware of any game that actively supports the Hero's Journey, but then again I still need to catch up on quite a number of games.

Well it's nice to see that your ideas are developing!
My best piece of advice now is to write up a small playtest document, test it with a couple of friends and tell us how it went in the Playtest forum.
As far as I'm concerned, that's the method that yielded me the most significant advances on my personal projects (ideas -> writeup -> playtest -> feedback; lather, rinse and repeat).

This way you'll be able to validate your modifications of the d20, or find concrete reasons why you might want to change to something different entirely.