Honestly I would try out anything that got away from the RPG elements laid out in WoW and tried something different like a Shooter, Space Sim/Flight, or anything that involves Real-time game play that doesn't rely on 'dice-roll' mechanics. Honestly I think a massive MMOFPS with combined arms (Infantry, Armor, Gunships, Jets, Artillery, Logistics, etc.) would be very intriguing. Also wouldn't mind something like an MMO equivalent of Freelancer as long as they tweaked it a bit.
Basically something where the developers/fans/players can't say "Oh it's like WoW except..."
Basically something where the developers/fans/players can't say "Oh it's like WoW except..."