Disable PvP - Enable Cross-Faction Chat - Make All Races 1 New Faction
Or make Not-Honorable kills count. If you have more dishonorable kills you will have more Threat in from mobs and guards and the range that it takes for them to notice you will increase more and more. If a player is killing people in a map, that player will appear on the World Map to all players from the other faction. If a player is killing low level players and he is killed by the opposite faction he may drop his loot to those players like a raid boss.
Announce on website that creating characters on Horde faction is available, if a player makes a Horde character, make that character BE Alliance but with the looks of a Horde character, make Alliance characters look like Horde to him and Horde characters appear as Alliance. It will seem like the player is grouping with other Hordes and playing against Alliance. Replace Horde and Alliance words on chat with each other so everything seems normal. That way, all the players that make Horde will think they are Horde but they will be playing Alliance without knowing.
Or when a player makes a Horde character on Outland, send him to Medivh.
Many theories as to how world peace could be achieved have been proposed but all of them are based on the past and thinking about the present world, none based on the future and thinking about the universe.
Stay inside dungeons and do not go outside, get out of the dungeon only to reset it. Level only by doing dungeons, it is not safe outside.