I have a notion that it's possible to have a "somewhat directed sandbox" game. I'm working on a combat/survival open world game with a procedurally generated tech tree to go along with the procedurally generated world. There is a "difficulty direction" (down) and the farther you go that way, the environment becomes exponentially more difficult, while the items/powerups become exponentially more powerful. (With the environment gaining power faster than the player, of course.)

This gives just enough structure for players to escape the Second Life trap. (What happens to people who have no needs/problems in a virtual world? They go right up the Maslow Need Hierarchy -- to Sex.)

Hopefully, this will provide the right kind of structure to let players game-balance the game themselves. (And give them enough scope to invent their way into and out of corners.)