WoW was the "Beatles" of the MMOs, it was a world phenomenon and gave something on a scale that had never been seen before. That's an incredibly hard achievement to repeat in what is now a diversifying and fragmenting genre. Only something genuinely revolutionary would rejoin the same player base and attain the same level of success that WoW had.
The MMOs on the horizon offer numerous technical improvements, higher production values, better technology... you name it. But can any single product deliver something so special that all MMO players will come across to it? They all offer an evolutionary step on top of what has come before but you can only experience that mind-blowing shattering of your innocence once!