There's been a lot of talk of games that will depose WOW as king of the MMO genre - Guild Wars, Lord of the Rings Online, Age of Conan, Warhammer: Age of Reckoning - but by and large they've come to nothing. Not that they're bad games necessarily (well, excepting Conan) but they lack a certain something.
The latest contender being touted is Aion, a Korean MMO that's getting a western workover (a process in which several people I know are working on, though I'm no more likely to get any insight from them as I do the people I know who work at Blizzard ...) but I'm sceptical.
The game that I think does pose a threat - due to the source material of the game and the pedigree of the developer, Bioware - is Star Wars: Old Republic. I came to MMOs through Star Wars Galaxies, and was gutted when that game fell appart, so I'm eagerly awaiting Old Republic (due, I believe, in 2010). The gameplay clips I've seen look good, but as ever with Bioware it's the cinematics that leave your jaw on the floor
; "Rotface down!")
; "Gnomes are scary")
; "More Gnome than you can shake a stick at")
; "The Big Gnome")
; "Aren't you a little big for a Stormtrooper")
; "Victory!")
; "Saurfang Down!")
; "Maebh Tuskarr")
; "Buliwyf the Tuskarr")
; "Andro_Tuskarr.JPG")