Blizzard's next MMO will be nothing like WOW yet will have many of the same elements and style. This tiem around Blizzard will opt for a higher graphical level because they know no matter what kind of game they will make people will play it, so making significantly better graphics will not drive away people but most likely lure them in. Their next MMO will obviously have no fantasy connections whatsoever, but they will not make anything in the present/past day, because that's not very interesting. Space looks like the way to go, with jumpsuits and Spore-like customization on your various races.
Blizzard will focus on some sort of instance based game because that is what they do best, but my guess is that they will put a lot of effort into dolling up the PvP aspect. WoW will always probably be the best raiding type MMO out there and they don't want to detract from that so their next MMO won't have raiding. Open world PvP is hard to get right and Mythic are the closest to getting that nailed so Blizzard's next MMO will have instance based PvP galour. Also they don't want to make it too familiar to EVE so that's another reason why open world PvP wouldn't be a focus.
The last thing is that Blizzard's next MMO will be easy to grasp and will just require a lot of time dedicated to it. This will be the big challenge. When you think 'futuristic' and 'space' you think 'massive' and 'expansive' and, dare I say it, 'complicated' making a space/futuristic MMO simple and easy to play will be a pretty big challenge for them.
So to summarise; Blizzard's next MMO will be space/futuristic.
It will have many different aspects to WoW but many similar ones.
It will have a different audience, probably a bit older and mature.
And it will be more focused around PvP.
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