I have always wanted (in any MMO with factions) the ability to change sides without having to change anything else. In the physical world, we have defectors, immigrants, and slaves who have become citizens. My Russian professor was an escapee from the Soviet Union. It was her lifelong wish to change factions, and she was so happy to be in the United States.
I know it is complicated design-wise, and I completely understand why Blizzard doesn't do it, but I think it would be perfectly logical that some residents of Azeroth would want to change sides. A human might want to live the simple life of a tauren. A blood elf may long to return to nature and attempt to fit in with the night elves. A gnome and a goblin might fall in love.
Some restrictions would apply. The Forsaken would be unwelcome anywhere but where they live now, of course. And once the defection was successful, you would not be welcome back to your homeland.
I don't think this should be as easy as flipping a switch or paying a fee. The defection should involve a painfully long and difficult quest series. It could begin with a sympathizer living on a Horde/Alliance border. You would have to build faction with the faction leader's race in order to be seen by a facilitator. And the final quest reward would be one of the faction leader's administrators allowing you passage through your new faction's lands.
Some races would never get above neutral for you, and you would have to work on getting past unfriendly with most of them. But the end result would be a troll being able to drink in the Beer Garden in Dalaran -- by herself and paying more for her ale.
If you could have a character defect to the other faction without changing race, would you? If so, what race and why?