A good amount of my close friends and I all play World of Warcraft but we have different tastes in what we like about the game. Everything ranging from what we like to do in the game, what classes we enjoy and what races we use. We always talk about the day that guilds could have both Alliance & Horde together. Especially with the two expansions showing The Alliance & The Horde together.
The main issue that I came across was reputation. I think I came up with the answer to have all races play together:
WoW: Plastic Surgeon.
The general idea is that you could pay, to change your characters looks. There is a program like that already, but this one would allow you to change the race. It wouldn't change your rep, only the race. And the races would be based on the class. So if you're a mage, you can only pick the classes that can be mage.