Just make all zones the way they are on Broken Shore - that the zone will adjust to your current level (but with a minimum level requirement to enter so you can't start a level 1 character in Pandaria). Then you can still level pretty fast but don't have to leave in the middle of a story based quest line. If you do all quests in a zone and reach max level in let's say Western Plaguelands or Burning Crusade already that's ok, you can still continue to do the other zones for gold an quest rewards (that should also scale up to max level).