Have it be confined to the plaugelands and have it Scourge vs Argent Dawn themed.
Players can actively join the scourge and transform into Scourge (like the WotLK scourge invasion however they die instantly if they leave the Plaugelands zones) but cannot turn other players into Scourge. Before the event starts players can go to Cult of the Damned members in the zones to transform into a scourge at the start of the event. While the event is active horde and alliance players are friendly. Horde and Alliance players collect Scourgestones for gear rewards at a AD vendor and players who are Zombies collect Fresh Flesh which can be used for different rewards than the AD vendor. The currency will be a debuff on the character like in the Blood Moon event which is changed into a physical currency at respective turn ins.
Players who decide to join the scourge become a ghoul with its own abilities. However as they gain more kills they can become an Abomination and later a Death Knight (WC3 inspired on horseback) which becomes more powerful.
The event should have a weekly quest which gives you a choice of one of 4 rewards. A rep token that gives 2000 rep to your choice WSG, AB, AV, or Argent Dawn factions.
The Argent Dawn rewards should be different gear slots than that of the Scourge to promote players wanting to play both sides also each vendor will have a mount and a AD tabard and a Scourge Tabard as well. The Argent Dawn mount should be a armored Horse with an AD banner on it while the Scourge Mount would be a great chance to add Valiance: