One thing that .Hack // Sign and the associated anime/games, in addition to Sword Art Online, is that virtual reality and MMOs is probably going to result in either our brains getting stuck in a virtual world or we all find ourselves in another dimension (Log Horizon). That may be the future, but right now we don't have a brain interlink system just yet, just a variety of various options to please our eyes and competing technology.
Yet, we spend so much time arguing about the future of MMOs, we ignore the fact that virtual reality is pretty much the inevitable destination for all MMOs and, in what I believe will be, the fourth age of MMOs (text based is the first, UO / EQ / etc. is the second, WoW is the start of the third), assuming that EverQuest: Next doesn't kick off another age in of itself.
MMOs and VR go together like peanut butter and bananas. The slower combat, larger worlds with more depth, and the social aspect (look at SOE's SOEMote) make VR very attractive to the MMO industry. Research has shown that our brain can accept VR as reality rather quickly, in so much that we will smile if an avatar smiles and treat what we see as if it's real, really upping the immersion. There is of course going to be a slow adaptation rate, most MMO players are super stubborn and set in their ways, but there is definitely going to be a huge market for it and that includes me.
I think there will be a big distinction between games like Diablo III and League of Legends in which they use top down or isometric angles and full 3D games, in so much that I do feel that large world games will be heavily desired for the VR environment. Of course, it's so hard to talk about it because the technology is nowhere near mature and anything you say is just guesses at this point. Various companies are trying to build their foundation as the one to be used by the massmarket and we can't even really begin to see what the future holds until the technology matures. So let's talk about the technology.
There is a lot of conflicting technology out there right now and a lot of companies trying to cash in on the impending virtual reality craze, while the technology remains in its infancy in the meantime. There are a few different paths companies are taking. Oculus, the industry leader, uses propriety components and software for the Oculus Rift. There are current spin-offs including the Gear VR by Samsung, which uses licensed technology and the framework for the Oculus Rift to make a consumer level device powered by the Note 4 (and only the Note 4).
Then there is the OSVR, the Open Source Virtual Reality group, which is various companies (Razer, Gearbox, etc.) that have banded together to declare that all VR should be open source. Currently the plan is to release "Hacker Dev Kits" which include all of the components to create your own device, with everything being open source and available.
Finally, Google has released Google Cardboard, a quasi-open source project that lets you turn most Android phones into a VR device using a cardboard box (that you can cut yourself), some various nuts and bolt type parts, and an Android phone. The upside is that it's probably the cheapest method, the downside is that those with Apple or Windows phone will have to pray that developers on those end integrate compatible technology with the Cardboard box, but most of the technology rests within the Cardboard SDK, which is included in both Android and Unity.
There are a few spinoff concepts, like Project Morpheus, but these are so properity that odds are there will be no market integration.
There are currently no consumer products beyond the Samsung Gear, which is currently getting rave reviews, and shows the promise of the devices, but otherwise the market is split into two camps: products aimed at game developers and consumers (Oculus) and products aimed at Hackers/Makers (OSVR), with of course Google Cardboard being a novelty in the arena (not to say the technology can't happen and be awesome, it's going to need more time, better displays, etc.).
We're pretty much at the precipice of VR entering the mainstream market, the deciding factor in everything is who or what is going to be the device everyone uses. The hope is that OSVR kicks off and there is one platform "open to all," but at the end of the OSVR comes off as an attempt to skip over dependence to Oculus, which got bought out by Facebook. Oculus has already made efforts to make a lot of their software open source as well, to alieve fears of market dominance and licensing fees. OSVR will need manufacturer adaptation if it's going to move from makerspace projects to the household, as well (although Razer is on board with making it).
On the flipside, having one line of consumer products all operating at the same specs can produce cleaner results, which is where Oculus comes in. The only issue is that Oculus holds the reigns of VR if it's what the market adapts and could cause some heavy issues in the VR market if Oculus is the product everyone uses. If Oculus decides something, then that something would be it, until the market readapted.
The best option is Google Cardboard, because anyone can cut out a cardboard box and anyone with an Android phone can hook it in (massive market reach), but there is some serious quality of life issues (i.e. wearing a cardboard box on your head), design issues (low end phone screens, while useable, aren't specifically custom designed for VR and motion blur, and pixel persistence is critical to avoid nausea), and of course it doesn't integrate currently with your PC yet (at least first party, third party apps are becoming available). Plus it doesn't come designed for the custom lenses that help immerse you into VR, even though anyone can make their own headset to stick a phone into, which will probably come out cheaper than the Gear (which is currently $199 just for the headset, phone not included). All of these issues are fixed with time, as phone screens continue to improve and the software and hardware matures.
Needless to say, it's the next "big thing" that I think we should all look forward to, because the most friendly thing to virtual reality is the ability to look around with your head and see others standing in front of you. I think that any MMO developer right now who isn't looking at VR as a serious contender for their games moving forward may find themselves left out of the fun whenever VR does hit mainstream.
Me, I'm personally excited about the prospect and will probably be daydreaming about it for a few more years before we start even hearing rumors about the "first full AAA VR MMO or VRMMO." Probably another good 30 years before we can transport ourselves into the game and be able to have some really cool //.Hack stuff going on.