Possible Solution Proposal
Introduce a new metric: Time-To-Cap (TTC) (Level 1 to max level)
Standardize the time is takes to reach level cap.
Apply this to every expansion that increases the level cap.
Shape the presentation of the MSQ around this time with questing, and cutscenes (perhaps still frames, with narration) bridging the gaps.
Of course, this should be OPTIONAL, thus allowing players to level in the manner of which they choose.
Unabridged, and abridged.
The Unabridged MSQ condensing nothing. The Abridged MSQ always conforming to the standardized TTC.