the glorious return of sandbox MMOs is coming sooner than you think.
World of Darkness
Life is Feudal
Shroud of the Avatar
all coming 2014/15
lol You can take ArcheAge off there, anyone who's played it will tell you it's one big theme park disguised as a litter box.
Anyway, you cannot have a successful sandbox without some themepark. People need goals and guidelines, whether they admit it or not. All you have to do is look at games like A Tale in the Desert or Face of Mankind to see how popular ( not ) player only content sandboxes are.
Make a good themebox or sandpark and you'll have a chance at success. Like Star Citizen, for example. It is more sandpark than sandbox, and that's one of its biggest draws. It actually has content for you to do, rather than for you to have to make it all up yourself.