Freedom to literally join/abandon factions. For example some Tauren may decide that he's done with the Horde and join some druidic faction neutral to both the Alliance and the Horde. Or someone could make Skryers/Aldor their main faction, thus becoming hostile to all players of the opposite faction. That'd be interesting...
Or another idea - one guild could declare a war on another (can be within same faction) lasting for a number of days, or until the enemy guildmaster surrenders, or until X amount of honorable kills are made. No guards would be able to interfere.