Challenges - Similar to FNV
I will admit this is a long shot, and a wierd idea, but it is a random idea.
But here it is, adding in challenges, similar to Fallout New Vegas, Kill a specific enemy provides Experience and/or Reputation of a faction, or even a Passive.
I am still leveling up, but that was already a thing on retail, when you killed certain NPCs your reputation with an opposing faction would go up such as the Timbermaw in Felwood or the Gelkis in Desolace. And when you run instances you get reputation in Outlands and other future expansion.
@Fugus I was more so thinking killing a specific amount of a specific enemy, for a one off bonus or a repeatable. Rather than every kill providing a reputation bonus. Kind of a dumb idea but it will be one way to make leveling through the first prestige, or level up a reputation, faster.