First time poster, and its a long one, read on if ready:
Its funny because the only "sandbox" mmos that are around are pvp based. I don't have a single pve-minded friend who still plays GW2 regularly. They all tried it, go to cap, did their wandering, and got bored and left. I see it as "endgame skyrim" syndrome. Wandering for the sake of wandering on a map you have already explored praying something new shows up.
The grand problem with all MMOs today is that new content takes too long to deliver. I remember monthly updates in Asherons Call, monthly events and minibosses and whatnot. Could you see a blizzard or Bioware doing that? Not with how complex the games are and how demanding the playerbase is. Minecraft succeeds because the "new content" comes from the players. Notch just has to sit back and police the place while the players add their own stuff. Bioware could get into that game by letting crafters design their own items to sell, but what people want is new bosses, zones, planets and story. On a time frame no company could meet.
Blizzard's original solution to the content problem was to make it difficult, but that game has been taken over by awful, awful players who want things handed to them and use their subs as some form of hostage, so they can plow through the content and claim boredom. Bioware went for story, an awesome one at that, and we have a 80+ thread on how everyone wants to spacebar through story.
People are right that there wont be a giant mmo, and thats how it should be. People have different tastes, and they play games for different reasons. I play both swtor and wow, but for different reasons. SWTOR does some things a ton better than wow (even with MoP) and vice versa. Some people like the pvp of GW2. However all the games have in common the playerbase that plows through everything without stopping to smell the roses then launch on the forums to whine they are bored. What happened to the days when MMORPGs were about immersing yourself in a virtual world?
I always have felt that the biggest problem with the gaming industry as a whole is that they have been bowing down to "fans" to the point its becoming design by committee and not by vision. Too many Bobby Koticks in this industry that care too much about the casual locusts that consume and move. Ask Zynga how catering to only casuals is doing. Developers need to go back to having this vision, and you can play in their world, or find another.