Bring out your inner game designer self and give opinions from your experience in other games on "how to make WoW more Addicting/Fun"
Here is my opinion:
All other games are addicting because of "Ranked Systems" so:
First i would invent a new type of item that everyone wants. Everyone must drool at the sight of said "new items"
Then i would implement a system where the population would be divided by Social Classes(LFR, Normal, Heroic, Mythic) and reward each class with a unique reward (new items)
This would give prestige back to the game but at the same time give a goal or a epic journey to each Social Class.
Maybe you like more the "new item" version of LFR, then go do it! After that you can try and get the "new item" in Normal mode.
EXPLANATION: The idea of dividing WoW in Social Classes or "Different Skill Levels" is essential to create a competetive environment
Everyone would be on a epic quest (according to their skill level) to get said "new items".
The "new items" must be something that EVERYONE wants. Make it as much snowflake as possible and must be impossible to get once the expansion ends to not flood the system with "awesome items"