Completing all the Story Progress Chapters of a zone should give you a hefty XP Bonus or Boost. This lets you speed up levelling a bit while still compelling you to play through the zones.

Just a thought. I love the new World Scaling system and it's great to go back and truly re-visit some of these old zones. But at the end of the day, I would rather experience 10 old zones per character while levelling two alts, than experience 20 old zones per character while levelling a single alt.(Edit) Here are a few examples of how it could be done:Give a buff for completing a zone, which grants an XP bonus. Let's say you complete the Jade Forest in Pandaria. Perhaps the buff reads a bit like an Achievement; "Hero of a Jaded People - You gain 100% Bonus Experience while questing in Pandaria (lasts 2 hours or until replaced)".Give a level-up token for completing a zone, that sits in your inventory and grants a level when used. "Pandaren Lore Idol. Consumable. Grants a level. Only usable in Pandaria. Usable up to level 89."Give a simple boost when the zone is completed, like a Dungeon XP Bonus.