Maybe like a couple articles I've read at mmorpg suggested, sandbox games are doomed to failure despite the huge desire in the gaming world to have someone make a good one. It's because of the 2 distinct types of gamers it attracts, whom when they meet, clash in an unbalancable manner and so all sandboxes fail. I guess the answer could be a double server sandbox in which some simple mechanic is allowed on one but not the other. I think I've seen a couple games do that in the high grade pvp genre.
In the end such a thing may be the only way to finally make a sandbox that actually lasts. Espcially since the dislike between the two types is so venomous. 2 opposite and not very nice views could hardly ever co-exist within the same game in a successful level. This may be why no gaming company with real money at stake has even taken this genre seriously or as a possible workable gaming platform.