After browsing some recent threads related to new classes and races, I got to thinking about the idea that some of you abhor and some love. New Race/Class combinations. This gave me the idea I'm sharing now.
Okay, so many race/class combinations don't make immediate sense in World of Warcraft's lore. While I personally disagree with this notion, I can understand some of the logic behind it. With that in mind, Blizzard could utilize some of their previous technology once more to give us something entirely different and interesting. Here's how it works.
You reach a certain level with one of your characters (for argument, I'm going to say 85). A host of quests now become available to you to explore. some of these quests take place in the greater world, while some are in normalized (to 85-appropriate ilevel) scenarios. During each chain, you learn more about a class that your race is not usually a member of. In the scenarios, you play a "member" of the class via a refined vehicle system (think LK quests where you are Arthas). By the end of that chain, you are tasked with bringing knowledge of the class back to your race's faction leader, who approves you to "train" others. You then choose whether to train your own people or another race.
You can also go on your own "class" quest and choose to then spread that training to another race, having learned how to teach your skills in a broader scheme. These choices are character specific, so you can keep passing on knowledge with more characters or you can change who you are "training" for a nominal fee (gold sink). You also wouldn't be able to buy a race change to a specific class until you've unlocked it with this process.
This unlocks the class for your chosen race as a "hero class," because your original hero is helping them learn more about it. Characters you build of that race/class begin at the higher level (85) and progress as they should from there. Unless blizzard ever decides to unlock all race/class combinations, this system would always be relevant in some fashion.
TLDR - Epic Quest Lines unlock new class/race combinations through character progression.