I anticipate that descriptions of realms will be much like the descriptions of planes (or at least a single layer of a plane) in books like Manual of the Planes for instance (3rd edition version. I don't know much about the AD&D version of the book.) That is; they may be hypothetically infinite, but in terms of described areas, they are geographically quite limited and circumscribed. There may be up to about half a dozen locations, described in a paragraph or two each. A few general characteristics of the realm. What kinds of inhabitants you can expect to interact with and how. In all, I don't imagine that these realm descriptions will amount to more than about two to four pages worth of text, total.
Then, I'll focus on what, specifically, I expect the PCs to need to do in realm. What they need to do to move through it and on to the next. What they need to do in the meantime to either earn their passage, or kill time until they can leave, or whatever the case may be.
Remember; I consider this to be a fairly "On the Road" type of adventure. Nobody goes to a new realm to set up shop permanently. No single realm is the campaign setting; rather--the entire cosmology is the setting, and the point is to sample many realms while moving from one to the other. Like episodic TV shows that featured moving protagonists--Wagon Train, Star Trek, Battlestar Galactica (the old one), etc. Or more accurately, Sliders is the model for which this will work.
I've got a partial list of realms that I will use. These are largely either planes from other sources (or more accurately, layers of planes from other sources) or other similar entities (such as planets in the Pathfinder solar system.) Most of them can be used more or less as is, but minor adjustments to fit them into the context of a "River of Worlds" type cosmology might be necessary. This is not meant to be exhaustive.
Realms borrowed from Manual of the Planes
- The Plane of Shadow
- The Ethereal Plane (blended with some elements of the Astral in the Deep Ethereal to form the Ethereal Sea)
- Lunia (first layer of Celestia)
- Ocanthus (bottom layer of Acheron)
- The Underland (also utilizing material from the Darklands book
- Avidarel (the dead void)
- Carrigmoor (post-apocapytic dome-city)
- Faraenyl (season based quartiles; fey and elf dominated. With some modification of my own.)
- Lizard Kingdoms (fewer unique critters. No bugs.)
- Curnorost (angelic tomb world)
- Akiton (maybe blended with Mars d20 and plain ole Barsoom)
- Gaping Maw (Demogorgon's Realm)
- Thanatos (Orcus' Realm)
- Androdynne (eladrin vs. demon war)
- The Demonweb (Llolth's realm)
- Yeenoghu's Realm (unimaginitively named, yeah.)
- Avernus (rubble-choked battlefield)
- Minauros (polluted swamp, with Jangling Hiter hanging above)
- Stygia (icey place. Combine with other icey plane ideas if needed)
- Nessus ("bottom" of Hell)
- Shadowsea (Dragon Magazine article on Dagon)
- Hollowfaust (city sourcebook from Scarred Lands of the same name)
- City of Brass (original concept is from 1,001 Arabian Nights, but I've got options on D&D sources)
- Demons in the Mist setting
- Ape Kingdoms
- Leng (although, yeah--borrowing lots of ideas from Lovecraft here)
- The City - the entire realm is a vast city
- Shattered World - floating worldlets in a vast atmospheric sea and flying ships
- Nephilos - realm ruled by fallen angels
All realms go into and out of conjunction with other Realms. Ideally, I'd like to have several options for PCs to pick from when looking to leave a Realm, but I'm not sure now much of that I'll do--if options are needed without additional preparation, well, that's when borrowing something already written goes into play. However, without doubt, the first realm that the PCs have to travel too (after leaving their home realm) will be the only realm in conjunction at the time that they wish to leave, so they'll be bottlenecked through it whether they wish to be or no.
Some of the planes that have been adapted into Realms for my game are heavily influenced by the alignment based Great Wheel cosmology from whence they originally came. There is no such thing in my setting as alignment--or, if there is, it'll be mostly marginalized to importance, and most creatures will read as "unaligned" by anyone who wishes to check. Yeah, yeah--this is more of a 4e concept, but it's my one concession to alignment at all. I think 4e should have gone all the way and chucked alignment, but I'm OK with a toothless version of alignment. If a realm is considered a Heaven (like Lunia) or a Hell (like Nessus or Thanatos) that's because of the nature of the place and its inhabitants, not because of its "alignment."
Similarly, losing this focus on alignment frees me up to ignore a lot of weirdness in D&D monster listings. What, really, is the difference between a devil and a demon in D&D? How about between them and other hostile outsiders like oni (from Oriental Adventures) or demodands, or efreeti or slaadi? Nothing, really--except the bizarre categorization of alignment and a handful of minor subtype abilities. This means that I can have much more "cosmopolitan" Heavens and Hells than D&D offers. Angels and ghaeles, or devils and demons, have little reason to consort in D&D, but much moreso to do so in my setting. Don't make assumptions based on what you think you know from D&D. It may not be valid here.