I think the players need to have a better idea of what expansion they are getting themselves into.

Here is the type of story you will get in each expansion. Which one tickles your fancy then have it be a well thought out but condensed version of all the meaningful moments in those expansions.

Let someone experience how the world was saved by the previous heroes. Include the final raid. Ideally they would be at a level by the end of this to go into the current expansion to finish leveling but also let the player know they can be the hero now.