DISCLAIMER: Brainstorm-y post - not very well organized.
1 - Progressive/Chronologically Dynamic World
When Cataclysm was released, the whole Azeroth changed for everyone - at all levels.
What if the World actually changed as you reach certain levels instead of whenever a new expansion is released?
What does this mean?
1 - Everyone starts in "Old Azeroth (pre-Cataclysm) " (even Death Knights)
2 - At level 58+ you will go to Outland (no change here, Azeroth is still pre-cataclysm)
3 - At level 68+ you will go to Northrend (no change here, Azeroth is still pre-cataclysm)
4 - When you reach level 80, you "advance" in the chronology of Azeroth, and thus see the Cataclysm event (cinematic, etc)
5 - From now on your Azeroth is the "New Azeroth" (post-Cataclysm), and all areas in it are meant for level 80+, including Elwynn Forest, Durotar, etc.
New Challenges / More Relevant Content
This allows Blizzard to fill the whole world with all sorts of new content - end game, rare monsters, challenges, secrets, events, etc - without interfering with the leveling experience of lower levels.
Expandable Concept
This idea doesn't have to be limited to Pre/Post Cataclysm.
Blizzard can later expand this concept to provide additional layers of content for areas relevant to any new expansion they release, such that the world evolves as you do, and you always have content meant for your level range in various places of whole world.
Q: How do I complete low level quests I left behind?
2 - Reminiscence (placeholder name)
Reminiscence takes you back to Old Azeroth so you can complete level 1-79 content.
A level 80+ can also group up with a level 1-79 to do 1-79 content this way.
Explanation:
Once you reach level 80 and witness the Cataclysm, you are in the New Azeroth.
Lorewise, at that point you already did everything you were supposed to do before the Cataclysm.
Gamewise, however, you will inevitably leave level 1-79 content behind you as you reach level 80.
In a way, you can consider that Reminiscence allows you to "relive" the past.
3 - Fight Your Equal
Ganking lowbies is not PvP - so don't give me the "roll a PvE server if you don't like PvP".
In my opinion, each area should have 2 Level-based phases:
Normal Phase - Max Level appropriate to questing in the area
Overlevel Phase - Level considered too high for the area
Example (Hypotetical):
Hellfire Peninsula Normal Phase = 1-68
Hellfire Peninsula Overlevel Phase = 69+
This means if your goal is killing lowbies in Hellfire Peninsula you need to bring your lowbie character.
How does this work when grouping with lowbies?
Q: Wouldn't this end up bringing high level players to Normal Phasing?
A: It would be the other way around - the low level player would be brought to the "Overlevel" Phasing.