Need help with a semi-homebrew cosmology idea

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I am working on a homebrew campaign setting and I am looking for some help regarding how best to approach the cosmology. I feel like fleshing out the planes, realms, and deities is a good place to start for a homebrew setting. I want to make sure that all these elements fit together well in order to create a sound foundation for the world in which my players will interact.

Here's my idea: I want to have a cosmology in which all the planes of existence are connected to one grand celestial tree, sort of like the old Norse/Germanic mythology surrounding the tree of life. I was thinking that the planes would simply "grow" and hang from the tree similar to how fruit would grow from a tree in nature. The only trouble with this is I want to use some of the deities and planes found in Greyhawk (like Pelor, the Abyss, Nine Hells, Elysium, etc).


Should I be concerned about doing this? I am kind of new to homebrewing so I don’t know what type of deity/realm lore might not fit with my “everything is on a tree” idea. And if this is a problem does anyone know how I might fix the outer planes mythos to better be attached to my tree? 

Thanks a bunch in advance!  

Well the lore of Yggdrasil (sp?), gives you a nice frame work to build your cosmology around. Within Yggdrasil all of the worlds/plains are within its branches and roots. Midgar, the world of Man, I think is in the center, but I'm not entirely sure. However we're just using this as a frame work so it's irrelevant. 

So here is a suggestion for you on how to make this cosmological tree fit in your campaign world. The branches of the tree can serve as connectors to all the dietific planes and serve as your world's version of the Astral Sea. Each branch could serve as the pathway to the specific diety's realm of influence. So one branch could lead to realm of Bahaumat, while another could lead to Pelor and so on. I'd suggest having the good to neutral aligned dieties in the branches and delgating the lower levels to the malaigned deities.

The roots of the tree could serve the same exact function as the trees branches, but for the evil and crule dieties of your world. Asmodeus, Tiamat, Vecna, etc. Again, each root would be the path to that specific diety's sphere of influence.

As for the Feywild and Shadowfell, you could have them be the different parts of the tree's trunk that head away from the Mortal world. With the Feywild, bright and verdent, being the path that leads up to the tree's branches and into the realm of the gods. Meanwhile, those that tread the paths of the Shadowfell are heading into a dark and dangerous place where the powers at work are uncaring and cruel. 

I'm not sure where to place the Elemental Planes. You could posibly put them radiating out from the Mortal Plane so as to show the deep connection to nature. The Norse mythology was big on the power of nature so this could work.

Hope this helps! 
Thanks man that helps a lot.  I just wanted to make sure this wouldn't wreck any prexisting lore and looks as if it won't. I'm definately going to start working more on it then!
You could also put the elemental planes as "bands" within the tree that run between Midgar and the higher planes. You could associate earth and water with the feywild (for example) and air and fire with the shadowfell (fire creates smoke...you get the picture) In 1e, the elemental planes were a covering around the "material plane" that separated it from the astral sea and outer realms. The "material plane" (Midgard) and the elemental planes were considered the inner realms.

I don't see a problem with blending cosmologies. This is just fictional and for fun after all. I've also created my own cosmology for a campaign that doesn't necessarily exclude the "established cosmology" of the Forgotten Realms, for example.