The WoW killer won't be an MMORPG. That market's pretty much been saturated by WoW already, and unless Blizzard makes a string of ridiculous business decisions, there's just too much barrier to entry for any other MMO game to rival its status as big fish. WoW isn't perfect, but it's done enough things right that even *if* a perfect MMORPG game came along, WoW can probably coast on pure momentum alone and still make a profit for the foreseeable future.
I've heard the "dynamic/generated" content theory before, and my opinion is that it's just a fad. The number one theory of the Internet is that if something *can* be turned into porn, it will be. Add that to the "myspace" factor, and you can imagine countless user-created zones with blinking dancing neon trees, loud obnoxious soundtracks, and weapon and armor made to look like flaming penises and huge boobs. Sure it might be fun for about 15 minutes (okay maybe more), but definitely not grandma-approachable.
What will eventually overshadow (probably not kill) WoW are probably going to be wireless portable web-based games like Puzzle Pirates or Pokemon.