WoW will never be beat due to the fact that it is the first mmo that was done right.
Anything after that will either be nit-picked for being different, or will be knocked off as a WoW wannabe in the eyes of mmo players. Finding that comfortable gray area between those 2 factors will not only be very hard for developers, but won't sell as well as WoW because of the fact that Blizzard got it right really early in the life of the child that is MMORPGs.
Millions of people play WoW.. Not because those millions of people were huge fans of Warcraft before it went MMO, but because of the simple fact that it plays well for an mmo.
Back when I tackled that beast, (Before it sold out with it's 500 expansions) I've only met a chosen few that have actually played any of the Warcraft RTS games, and they just happened to be roleplayers.
Changing the style to fit a new mmo won't sit well for mmo players because it doesn't feel like WoW, and thus won't sell well. Prime example is Age of Conan's combat system... Hated by WoW players because they've grown accustomed to pressing one button to initiate a skill or spell.
Using up to date graphics won't work because although they are up to date with technology, it's safe to say that 90% of gamers aren't.. Thus the game won't sell well because in current times of recession and constant job-finding.. We won't have the money or time to update our rigs to even remotely get the game running passed that little updater screen.
Prime example is the upcoming Star Wars MMO... It sounds like it has a new innovative way of playing an MMO but will most likely fail to surpass WoW due to the fact that WoW was capable of being played on a computer I used to have that I had bought back in 95...
Making an MMO that is playable on older computers and resembles WoWs' style may be their best bet, but again still falls under the fact that WoW came first and thus anything remotely similar will be pawned off as a Wannabe in the eyes of WoW fans or any MMO player that's aware that WoW exists.
It's not that WoW is the MMO that beats all MMO's because it's the best MMO in the history of MMO's... It's that Blizzard got it right extremely early in the race and now every other developer has to live up to its name...
If any MMO had a standing chance against WoW's superiority.. I would have to say it would have been Star Wars Galaxies, but alas, Sony fucked that up royally when they decided to blow off their loyal customers to try to bring in a new fan base that they didn't need... This is a prime example of why Developers shouldn't cave into people who complain that shit's too hard in the forums.