Star Wars: The Old Republic writing director Daniel Erickson has stated in an interview that someone will surpass World of Warcraft someday, but it could take a very long time, maybe 20 years.
"Great games grow the market, and the market grows over time," said Erickson. "These are the only truths that we seen continually go. So yeah, I absolutely think somebody will be bigger than WoW someday. Whether that's next year or in twenty years from now, who knows."
Erickson also thinks the MMO world can handle more than one World of Warcraft-sized mega-successful MMO: "Absolutely. And perhaps not today, and perhaps not...maybe not right in that size. But you can definitely have, I think you'll see ones in the future that break a little bit more along territorial lines. I think that you'll see ones that break more along age lines and stuff like that."
Star Wars: The Old Republic looks to me a major player when it's released, hopefully sometime in 2011.