So, this is pretty drastic but hear me out.
Suppose WoW was down to one playable faction. And I am not an Alliance fanboy, by the way. I just think it would make more sense to disband the Horde given their allegiances hang by a thread as it is in Cataclysm. So, suppose Blizzard removed them.
- It would make the story a lot more focused. Right now there is so much going on with everybody, it's nearly impossible to keep track of it all.
- People wouldn't argue and flame over which faction is better. Frankly, I hate all the hate on different WoW forums including this one.
- There would be no more ganking. For those of us who enjoy leveling alts, that is a huge plus!
- More people would be online that you can play with at odd times, which is another big plus for me personally.
- The game would be twice as big for everyone! This is the biggest thing of all. If all the developers focused their efforts on one faction, we'd have twice the quests, twice the everything instead of just one half of the game per faction.
On a technical level, I am not a programmer but we already have faction transfers and same-faction arena matches. That is most of what would be required?