With the new leveling system blizzard have made for Shadowlands, We will be able to pick a certain Expansion to level our Char. This is great so we can experiance the entire story of Outland Northrend Cata pandaria etc etc etc.
The only Problem is that the player wont be able to finish the story because it happens in Raids. I think it would be ideal for players to be able to finish the story if they wish. And my solution to this would be a NPC Raid for the Zone you have chosen.
This is content you shouldnt be forced to do, but you can if you wish to finish the story of the expantion.
To make it fun you could be givin certain missions for your spec/class within the raid in order to progress to the next boss.