Blizzard hasn't talked about what it plans to do after Shadowlands. I speculate that the longterm solution would be to do this every expansion—so in the next expansion, Blizzard might move Battle for Azeroth into the multitude of choices for new character-leveling experiences and push level-cap characters back down to 50 to start whatever expansion comes after Shadowlands—an odd hybrid of traditional MMO and a seasonal model common in plenty of modern multiplayer games.
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).
Further, once the new character has completed the starting zone, the player can then choose to level up to 50 in any previous expansion of their choice—vanilla/classic, The Burning Crusade, Wrath of the Lich King, Cataclysm, Mists of Pandaria, Warlords of Draenor, or Legion. The expansion's content will scale perfectly with the character level, and each is tuned so that reaching level 50 involves playing through the content at a similar pace to a similar level of completion as those expansions offered when they were current content.
A new army of the dead rises: to combat Sylvanas' assault on Icecrown, Bolvar Fordragon— the revered former paladin who took on the mantle of Lich King to keep the Scourge at bay—has raised heroes from among all the peoples of Azeroth to bolster the Death Knights of Acherus. With Shadowlands, pandaren and all allied races will now be able to become death knights.