In my previous campaign, the Great Wheel went:
Celestia (riverhead of Oceanus, which flowed from the Silver Sea),
then Arboria (nurtured by the life-giving river),
then Ysgard (where the armies of Good trained, and fought for sport),
then Faerie (which replaced Limbo as the representative of Chaotic morality).
Below Faerie were the tunnels of Pandemonium, though that wasn't a separate plane -- rather it was simply the Abyss trying to get a foothold in Faerie.
Faerie, you see, had made a bad deal long ago, and as part of their remuneration, they diverted a portion of the flow of the holy river Oceanus from returning to Celestia. This diverted portion flowed into the Abyss (allowing the abyssal incursion), where it became tainted, and the outflow is called the river Styx.
The river Styx "begins" in the Abyss, flows through the top layer of Carceri, cascades down the burning slope of Gehenna, and then traverses every layer of Hell. The river Styx ends in the pit of Nessus.
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The upper planes form a sort of circle, which could be characterized as "Justice (Celestia) -> Nurture (Arboria) -> Righteous Strength (Ysgard) -> Passion (Faerie)", and Passion feeds back into Justice.
The lower planes could be characterized as "Corruption (Abyss) -> Imprisonment (Carceri) -> Malice (Gehenna) -> Death (Hell)", with Passion feeding Corruption, and Death ending the flow.
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In my cosmology, Slaad came from an "Astral chaos" which was kind of a buffer between reality and the Far Realms, or perhaps it's the churning froth of reality's wake as the prime passes through time.
The Elemental planes were all one place, which could be seen as a circle or an infinite plane, where Fire fed on Wood which fed on Water, etc., and had very few permanent locations (except places which could fly and thus stay in their favorite elemental climate, like the City of Brass).
Ghosts hang out on the Ethereal so long as someone on the Prime remembered them. This is why we must always honor our great ancestors and learn the tribe's songs. The spirits of the forgotten? Who knows.
Cheers, -- N