Disclaimer:
I am not saying that it is certain that 4th specs are the next feature in WoW, only that it is the most sensible. In truth, I have no idea what's coming in the next expansion, but if I had to guess, it would be 4th spec. Read on to find out why.
Introduction
In November, we're going to find out what the next expansion will be based on. Considering that success or failure of Cataclysm and MoP, we can only speculate what the next expansion will be about. However, one thing that is true is that every expansion, WoW introduces a new race or class into the game.
In MoP, Blizzard took an interesting turn; They introduced both a race and a class into the game. This makes the next expansion's content even more intriguing because it breaks the cycle of game additions that Blizzard has followed since the Burning Crusade.
So what's coming next? My guess is 4th specs. Here are the 5 reasons why I believe that such a development is not only possible, but the most logical direction for Blizzard to take the game.
1. Additional specs would rekindle interests in the old classes.
The addition of new specs to the existing classes would make these old classes more interesting to a wide swath of people who are potentially no longer interested in WoW. Most of these specs have existed in their present form for almost a decade. A new spec would not only reinvigorate those older specs, but the entire class as a whole. For example, the Shaman player who grew frustrated that Blizzard wouldn't allow Shaman tanking or 2H Enhancement could come back to the game if such an option was offered via 4th spec. Furthermore, an additional spec could add a level of complexity or variety that is currently missing from WoW but is present in other MMOs.
2.Any new race or class addition would eclipse the additions from MoP.
Monks and Pandaren were recently added to the game via the MoP expansion last year. As of this writing (March 2013), they are still the least played Race and class in the game. Any new race or class additions would cause this status to continue. It's not a huge problem with Pandaren, since having a least played race doesn't really effect things much, but the Monk class surfing the bottom of the heap is a very big problem, since it is a new class that simply hasn't taken off for a variety of reasons. A new class, or especially a new hero class could make that status permanent, and cause a lot of problems for the class in terms of balance and future class design. In short, Monks and Pandaren need their time to grow into their new roles as a new race and class in WoW. Introducing a new class or race could undo all of that, and make their implementation pointless in the long run.
3. Lack of new races that can be sensibly implemented into the game.
Let's face it, there just isn't many choices left in terms of a future WoW race. I've seen some people suggest Ogres and Nagas. Honestly though, those options simply aren't realistic beyond fanboy fantasies. Most race suggestions come with loads of problems, both in terms of gameplay and lore itself. Blood Elves, Draenei, Goblins, Worgens, and Pandaren were all OBVIOUS future races for the game. There simply are no other races that are on that level of popularity or inevitability. Blizzard could just make something up, but seriously, who would really want to play something that Blizzard just pulled out of its butt for the sake of an expansion?
Also let's not forget that Blizzard is currently working on remodeling the Vanilla races.
4. 4th specs fill holes so that we don't need new classes that overlap with existing classes!
Yeah, I want a Demon Hunter class. I would love to see a Demon Hunter class. There's two ways Blizzard could do a Demon Hunter class; Either create an entirely new Hero class into the game, or make it the Warlock 4th spec utilizing Dark Apotheosis as a template. The latter is a lot easier, and will (over time) satisfy Demon Hunter fans whether they want to admit it or not. People want a Necromancer class? Death Knight 4th spec. Blademaster? Warrior 4th spec. Warden? Hunter 4th spec, and on and on. Implementing this into the game plugs up the class demands, and allows players to finally get those Warcraft 3-inspired classes they always wanted.
5. The end of the pure classes.
Is there a reason for pure classes anymore? I have yet to see one logical purpose for their existence in modern WoW. They simply no longer serve much purpose except for clogging up the LFD queue. Also, Hybrid specs are not only competitive, in some cases they're actually doing better at the DPS job than the pures themselves. Throughout Cataclysm, two of the best DPS specs were Balance Druids and Shadow Priests. I bet it made Mages happy that the Druid who could also tank and heal could also out DPS them as well. In short, pure specs make no sense in modern WoW anymore.
Not only are hybrids nipping at their heals in the DPS department, but pure specs are loaded with utility spells anyway. So why not just give all the pure specs a hybrid 4th spec and allow them to join modern WoW? It just makes sense at this point, and adding a 4th spec wouldn't interfere with the other three DPS specs. So if you love your Rogue DPS specs, you'll still have them, but now you'll also have a healing or tanking spec that would make your Rogue much more valuable in groups, and much more popular.
Conclusion
Obviously, this could all be wrong and something else entirely different may be planned for the next expansion. We'll know in 8 short months. However, I feel that Blizzard would be missing a big opportunity to revitalize their game if they decide to not go with the 4th spec concept. The Monk and Pandaren implementation proved that people still love their old classes, so reinvigorating those classes would be a great way to secure the game's life for years to come. I know of several ex-Shaman players who would resub today if Blizzard announced a 4th spec Shaman tank.