Another option for new content: factional class flipping?

Last week I talked about mid-level content and as expected, many of you expressed an opinion. One comment in particular from autumnbear got me thinking. Autumnbear writes:

"How about updating the overall lore for the game, and allowing more class choices for the current playable races. Still waiting for the Night Elves to teach my Human how to be a Druid, especially since he's Exalted with both Darnassus AND the Cenarion Circle."

Now that's something I can chew on. What about quests that will allow you to make race/class combinations that are not otherwise available in the game? I think this is a tremendous idea if the process is sufficiently difficult to prevent everybody and their uncle's cat from doing it. The original Everquest had epic quests that took some players months to complete. Everquest 2 had the betrayal quests which allowed you to change factions which took a really long time to complete.

Why shouldn't a human, in Autumnbear's case, who has earned significant faction with Darnassus not be able to learn the finer points of being a druid? Gnomes have no healing classes. Why not create a very difficult quest line for those gnomes who are exalted with Stormwind that would allow them to become a paladin or priest?

I know I'm going to get a billion comments about game balance. Admittedly this would likely not count as "mid-level" either, but I think that if it was properly implemented it could be a valid source of that new content that everybody is craving.