Introductory events involve the (level-capped, or close to it) player running errands for the Sha'tar. (Make Outland relevant again!) When the expansion launches, a portal opens—preferably from Shattrath—to new makeshift settlements on the last relatively uncorrupted continent of Argus. By way of a phased event/quest chain, portals are eventually added in capital cities.
Players are sent directly to their faction's first base camp. The Horde sees Argus as abandoned property and as a new frontier. (In fact, ideally, the Horde intro quests don't explicitly identify it as Argus—just as "a Legion-occupied world that can possibly be saved.") The Alliance, however, sees it as draenei territory to be reclaimed. There are two or three new reputation factions that are Alliance-only (one led by Turalyon and Alleria, since they need to show up again), an equal number that are Horde-only (one of which is druidic and led by Bashana Runetotem, who's been upgraded to boss-level), and a few that are neutral (including a Sha'tar splinter group and a Broken faction).
The "safe" continent has been under the protection of some external force all this time. It's still very damaged and corrupted—probably to the same degree as Zul'drak, for the most part, but likely with some zones more like the Eastern Plaguelands or Shadowmoon Valley. (Of course, this is only in terms of overall feel—they'd be original zones, and not too reminiscent of the zones to which they've been compared here.)
Most enemies are demons and mutated wildlife. The demons include some old standbys, some variations on themes, and some completely new varieties. (Many of them would likely be eredar-kin of one form or another.)
Beasts would display varying degrees of demonic corruption, represented by different skins (diseased, scarred, fel-touched) and possibly slightly different models (spikes, et cetera). Any completely uncorrupted wildlife would either be rare spawns, or be limited to specific subzones. Argus-native animals would generally have disparate limbs (i.e. paws in front, hooves behind) and possess facial tendrils. (Make the draenei look like they belong there!) New hunter pets are available; some are variations on existing pet families (considered cats, bears, birds of prey, and so on for purposes of taming and care); others may well be entirely new. (This would be a perfect place for a cat—either rare mob or quest objective—that's a shout-out to A.E. van Vogt's Coeurl.) New mounts would be available at exalted reputation with certain factions.
Either at launch or in the first major content patch, the ancient eredar capital Mac'Aree becomes accessible. It is a multi-winged dungeon complex—including at least one raid with 10-player and 25-player heroic and normal modes—and is a zone unto itself, as opposed to a subzone of another region. Talgath, Hand of the Deceiver is camped out somewhere in Mac'Aree, smirking at players' petty efforts. And whenever players finally get to wipe said smirk off of his face...he'll let them know, with his dying breath, that Kil'jaeden is not going to be happy with them.
Demons start spawning outside capital cities and the occasional town. This should be at least as annoying as the Zombie Infestation: Even early on, they start to make things difficult for everyone if left unchecked; in later phases, they start building little bivouacs that have to be actively destroyed by players. When killed, they drop items that can be brought to representatives of relevant factions. (The Alliance faction is draenei-led, obviously; the Horde faction is orc-led and really wants to get back at Kil'jaeden for the whole corruption thing.)
The initial portals to Argus are located in Shattrath and maintained by the Sha'tar. They send players to their initial faction bases, which are located in the Tanglewood (unless there's more than one entry point, which is a possibility).
One of the lowest-level new zones. Think a combination of Sholazar, Felwood, and Zul'drak in terms of feel. The base camps here are essentially "tent cities." Enemies here are beasts, Lost Ones, plant elementals akin to lashers, and various minor demons.
Hunter pets in the Tanglewood fit into the cat, bird of prey, and bear families. Argus-native "cats" have raptor-like claws for hind feet; birds have taloned wings (like an Archaeopteryx or a baby hoatzin). "Bears" have somewhat longer tails than the existing model; while their front feet are standard bear paws, their hind feet are somewhere between claws and hooves.
The Great Savannah
May also be an entry-level zone. Vaguely reminiscent of the Barrens, pre-Cataclysm—grassland with a few stands of scraggly trees. Not only are there demons here, there's also at least one weird demon cult. Either one faction per side, or a single neutral faction, sells special mounts at exalted reputation.
Most hunter pets in the Great Savannah fit into the wolf, spider, and rhino families. Argus-native "wolves" are similar to saber worgs and have a distinct ridge running down their backs; "rhinos" look something like a kodo crossed with a styracosaur (and might also be available as mounts). Spiders are thick-limbed and fuzzy like the existing tarantulas, but sufficiently different as to merit a new model.
This is one of the two places where you'd most likely find Hemet Nesingwary. You know the drill.
Not a world PvP zone (although it has objectives to capture); nonetheless, Anchorite Tessa's memories of this region were fairly accurate. Relatively unspoiled; however, that's on account of it being harsh enough that demons don't even want to live there. Adventurers are less picky.
Most enemies here would be beasts; Legion activity would be fairly minimal, although there'd certainly be corrupted air, water, and ice elementals.
Hunter pets in the Kaarinos Mountains could potentially fit into any of several families; there would definitely be foxes, with ridged backs and splayed-claw hind feet. Because of all of the beasts, this is the other likely place where Hemet might be found.
The Felcurse Bayou
The nastiest, creepiest swamp you can imagine. (Draenei NPCs in this zone make sardonic jokes about getting stuck in swamps all the time.) Players have to swim, or at least cross bridges, a lot. At least one town is on a huge stationary barge.
Enemies here are similar to those in the Tanglewood, although the specific beasts and demons are different; for example, there are creatures akin to threshadons in some of the deeper lakes. Most hunter pets in the Felcurse Bayou fit into the crocolisk, chimaera, carrion bird, and turtle families. "Crocolisks" have kentrosaur spikes; "chimaera" somewhat resemble two-headed Chinese dragons; carrion birds use a variation on the raven model, only with clawed wings; and "turtles" look something like mata matas.
The Silver Vengeance
Led by Turalyon and Alleria. Mostly paladins, of all available Alliance races.
The Runetotem Expedition
The Cenarion Reclamation
Another Cenarion Circle splinter group. Made up primarily of druids of all available races.
The Sha'tari Vanguard
Sha'tar splinter faction. Mainly draenei.
Mac'Aree Dungeon Complex
Mac'Aree is probably not on the "safe" continent, although it may be on an island off the coast. In its heyday, this was a city similar to Shattrath in terms of architecture, albeit built on a much grander scale and made of paler stone. Although still basically intact, it is now a massive and ominous eredar fortress with felfires burning everywhere. Two huge elite wrathguard sentries stand guard at its gates (of which there may be more than one set), attacking anyone foolish enough to wander too near. A safe haven—run by Broken affiliated with one of the neutral factions—exists somewhere in the complex; there, repairs, refreshments, and reputation rewards are available. (There might even be a flight path, available after a phased event is completed; however, it's probably not in the complex itself.)
Mac'Aree is separated into (at least) the work district, the outer arcane district, and the city center. (It's probably laid out in rings, much like Shattrath.) The work district pits players against various sorts of mo'arg, mechanicals, and Lost One slaves. The outer arcane district would be inhabited primarily by petty eredar (wrathguards and lesser caster types) and the occasional roving pack of tamable warp stalkers; this is where the key to the city center would most likely drop.
Talgath would be the final boss of the city center; however, his defeat would not be the last we'd see of Mac'Aree. In a later patch of the expansion, the Spire—a huge and formerly inaccessible floating building—opens. This is where players officially face Kil'jaeden (although even this latest defeat may be merely a setback).
This is entirely fan-made. However, if Blizzard were to decide to appropriate these ideas and build upon them, I'd be absolutely jazzed: Playing through this would be reward enough in its own right.