There will never be a WoW-killer, in my opinion. I think all game developers, including Blizzard, need to just give up on the idea of ever topping WoW. WoW's success was a perfect storm of game design, market desires, and fortuitous timing. However, the market has evolved considerably since the peak days of WoW's success. Much of the market is probably suffering MMO-burnout from the massive glut of self-proclaimed "WoW-killer" games that constantly failed to fit the bill. Further, as the market has aged, the time and effort demands of an MMO with such epic scope as WoW has become difficult to support. Even the hardcore segment of the gaming community seems to be moderated compared to its earlier days, considering the quick decline of Wildstar, an MMO which very explicitly targeted the hardcore contingent.
To use an analogy, at the height of WoW and the parade of self-proclaimed "WoW-killers", gamers feasted on a smorgasbord of epic games, all requiring massive, dedicated effort. It was not unheard to have some people playing 2, 3, or even 4 or more such games at once. The level of "hardcore" from some of the players in these games was just insane (I was once a guild-leader and had someone apply to my guild who had quit his job so he could spend 50 hours/week playing WoW). However, after years of feasting, reality, in its typical ponderous and inevitable fashion, began to impose itself and make known to us that our capacity for the feast is not infinite. In the aftermath of all that epic feasting came the most epic case of indigestion ever known. Gamers become a kind of "violently ill" from just basic overeating. Consequently, gamers have become revolted at the idea of having to engage in such a massive gluttony ever again.
In my opinion, game developers need to find ways of allowing games to be played at a more moderated pace rather than require the maniacal, sub-basement dweller game-addict type mentality. Instead of these epic feasts as before, the games have to be more moderate. The portions need to be more bite-sized and more easily digested. Not so much to the point of being like baby-food; the meal still needs to be nice and meaty. It just has to not require one have the digestive track of Hercules to have any hope of coping with the meal.
This also means, in my opinion, that game developers need to give up on the idea of their particular game being that one and only game that everyone is playing. Instead, any given game probably should focused toward appealing to a particular cadre of fans or a particular collection of tastes and interests. In my opinion, trying to design a game to appeal to everyone will just ensure that it appeals to no one at all. Even WoW, at its peak, didn't appeal to everyone, just a very large contingent. Knowing your audience, in my opinion, is extremely important. It's easy to become highly ambitious and seek to conquer the Universe; however, that ambition needs to be reigned in and tempered with a more moderated approach that fits within one's given time, resources, and capabilities.