You mean just like how that happens now?
If you are leveling a new toon in Warcraft at the moment you only see a fraction of the game anyway. But instead of leveling through a whole expansion like Blizzard is going to let you do in Shadowlands right now you get bits and pieces of every single expansion there is. Which is not only confusing as hell for new players, but it is extremely annoying for players who want to enjoy the story, but still be able to efficiently level.
So what Blizzard has done is taken the disjointed hodgepodge mess that players experience now and brought order to the chaos. Not only that but they have done it in a way that not only makes it easier, but provides players with a choice of where and how they to do it.