Looking forward to hearing your ideas!
Keep in mind that the EXP required to level would also be increased with this proposal. So going from 1-60 in Azeroth would take almost twice as long than it currently does on live (to offset the fact that there are only 60 levels rather than 110). So players would be staying in each zone much longer than they currently do. I think "grey quests/mobs" would be highly improbable. In fact, I'm more worried about the opposite (not enough quests in some zones).
And as for 'disjointed leveling', I think ultimately it just comes down to the responsibility of Blizzard to make it clear that Eastern Kindgoms/Kalimdor 1-60 is one "story block", and BC --> Legion is another. Imagine something like this in the Adventure Journal when a player hits 15, but 10x prettier:
http://i.imgur.com/z15EbnB.png
That makes it clearer to the player that Eastern Kindgoms/Kalimdor is one "story block", and that BC --> Legion is another "story block", and that starting the "Progresive Story" means not continuing with the "Classic Story".
If new players do decide to go to Outland or Northrend at 15, I think that the game should highly encourage them to go to Cata/MoP at 30. They've effectively "chosen" that "levelling path" and the game should make it clear that they'd have the best leveling experience going to one of those zones next. Something like "Continue your story here [Cata/MoP]. "But if they want to return to Azeroth, that's their prerogative.
What do you think about that?