The experience curve is adjusted to make it faster than ever to prepare for the newest challenges, and each level along the way will provide more meaningful increases in progress and power. All expansion zones are getting more flexible, too; they'll be able* to scale to you and your friends while you level to 50, so it's easy to try a zone you've never played.
Blizzard started testing World of Warcraft: Shadowlands last week, and is slowly allowing players in to test new zones and features. While the “Adventures” mission table isn’t available on the test realm yet, we should see some version of it pop up in the next few months. Whether it actually turns out to be an autobattler or not remains to be seen.
In previous expansions, entities like the Loa of Death Bwonsamdi, ex-Forsaken Queen Sylvanas Windrunner, and corrupted titan watcher Helya all have spoken of the Shadowlands, and for some, the desire to conquer it. Sylvanas' character arc throughout Battle for Azeroth ends with her seeking to control death itself, indicating that she will play a pivotal role in Shadowlands.
The Shadowlands can be seen as a shadowy version of the physical world, and can be routinely visited as a spirit when player characters die. Governed by the Covenants, the four ruling realms of the Shadowlands are Bastion, Ardenweald, Revendreth and Maldraxxus (although countless others exist), which serve as the main setting of World of Warcraft's eighth expansion.