There’s plenty going on, including a leveling squish. Rather than increase the maximum level as every expansion has, they’ll be lowering it all the way back down to 60, the original level cap back during Vanilla.
Another massive change is the implementation of Chromie Time, which will allow players to pick which expansion they wish to level through. With so many zones and now so few levels, players won’t be able to level through them all on a single character anymore.
But which of the expansions is the best to level through? The obvious answer is your preferred favorite, but all things considered, there are time differences between the many. Wowhead recently released a guide with the average hours it’ll take you to finish each.
The fastest is Warlords of Draenor, which takes an average of 12 hours. With massive bonus experience objectives around and rather compact zones (especially with flying), it’s easy to see what makes this the fastest.
— Wowhead (@Wowhead) October 8, 2020
Legion comes in second at 14 hours on average, though the expansion has been massively gutted due to the removal of Legendaries and Artifacts. Still, plenty of bonus objectives and treasures provide quick experience.
Battle for Azeroth comes third at 16 hours, and has the perk of being recent enough to have more honed gameplay. Mists of Pandaria is tied at 16 hours as well, dragged back a bit by dialogue and narrative-heavy questlines.
While these four are your quickest bets, the others all remain viable. Cataclysm takes about twenty hours, but much of the expansion will be unreachable due to Chromie Time automatically ending at 50 and booting players out before they can reach zones like Uldum and Deepholm.
Wrath of the Lich King rings in at 24 hours, tied for last with The Burning Crusade. Due to being the oldest, there are plenty of quality of life improvements missing from both, questlines taking quite a bit longer and many involving more fetch-style monotonous tasks.
It’s important to note that the times given here are averages for what most players are going to experience when moving through solely with leveling in mind. Speedrunners will undoubtedly find quicker ways to reach the 50 mark before Chromie Time ends, and those that are taking a more casual approach are best served picking the one they enjoy most rather than attempting to scramble through as quick as possible to find their way to the Shadowlands.