Let's be clear where I'm coming from, here: I've been playing World of Warcraft almost continuously for 15 years. (I lapsed for a big chunk of the Wrath of the Lich King era for IRL reasons.) While I understand the appeal of WoW Classic, I am very much a proponent of modern WoW; the game has evolved over time to be more accessible for me to play just a couple of hours a week, as real life now dictates. But I tend to play solo or just with one or two friends on a sporadic basis, which means I'm looking for something very different than the hardcore raiding or PvP crowd.
Keep raising the level cap but crunching how much experience is required per level as you do. This is the approach WoW has historically taken. But as this happens, levels start to seem meaningless. You gain them ultra-quickly, and only a smattering of them actually introduce new abilities or other perks. It also means that you blast through the leveling content at such a pace that you never finish individual zone stories; you only finish a small portion of an area before you have outleveled it and are ready to move on.
During the Burning Legion's third invasion, the Knights of the Ebon Blade traveled to the Shadowlands to acquire the Aggregates of Anguish, five tormented essences of powerful souls - Admiral Proudmoore, Soulbinder Nyami, Grand Empress Shek'zeer, Grand Apothecary Putress and Archmage Arugal. Salanaar required these to conjure the Steeds of the Damned for the order's new Horsemen.
It's hard to get a sense of an entire World of Warcraft expansion from 30 minutes of play. In my short demo, I recreated my main (a Beast Mastery Hunter) and played through the introductory quests from the Bastion zone, a heaven-like plane from Azeroth's afterlife. The minute-to-minute gameplay very much followed the formula of modern WoW questing, with a variety of tasks like challenging specific enemies to duels, helping craft things in timed minigames, and of course, killing 10 rats (well, not rats, but they might as well be).
The Shadowlands were originally intended to be a fully playable zone in World of Warcraft, designed for high-level play. The idea was that as deceased characters made their way back to their corpses, they'd see high-level players battling creeps in the area, which would inspire lower level players to increase their character level so they could experience the zone for themselves.
The spell called "spiritual projection" or "astral projection" brings the caster to the Twisting Nether instead of the Shadowlands; and the Twisting Nether is also referred as "realm of ghosts". However, both planes don't appear to be related, as Outland is floating on the Twisting Nether, but the Shadowlands can be reached in Outland in The Burning Crusade.