Nothing will replace WoW.
Blizzard has, as it is known to do, cornered the market on products that take a niche environment, added an incredible ease-of-use, mainstream element, and made it their own. Millions of non-MMO players have picked up the game simply because of the simplicity and peer pressure, and it's spread like wildfire. Since it was their first, WoWheads will always compare other games to it, and the more in-depth rpgs (SWG, for instance) will begin to pander to that audience because it's HUGE.
Yes, I'm a hardcore gamer in that I like complexity, not in that I spend 40+ hours a week online (which, sadly, I do... it's for work, I swear! ^_~)