How about taking a game with the level of interactive everyday stuff of The Sims and turning it into a kind of adventure game. Give the player direct control over a character and give them lots of freedom to interact with things, then give them a specific set of locations with a story and some goals to achieve. Ie. Like the "solve the mystery of the haunted mansion, and work out how to exorcise the evil contained within ." type of game / film. So the player would have a menu with the kind of Look, Push, Pull, Mop etc etc. keywords that standard adventure games have. And they would have to go around looking for clues and getting out of locked rooms etc. Instead of having a game based around shooting, they would have to do a whole series of actions to get to a result. Thus making a STORY based around ACTIONS. Ie. The hitman kills the bent copper, hides their body in the boot of his trunk. Mops up the blood, Locks the door and drives to the crusher (bleurgh!). The scientist sees the raiders, crawls behind the table to the toilets, goes out the window, climbs down the drain pipe, runs away and calls the police. The guy buys some flowers, leaves his house early to make sure he isn't late for his date, apologises for the dumb things he said, and goes out and has a good time. :( So the story isn't about the END RESULT, ie. the guy in Deus Ex rescues the hostages, but it is about a SERIES of ACTIONS that lead to the result. Ie. It is all part of the story. Not just what happens at the end of the level.

Isn't that every text adventure ever, or Day of the Tentacle?

Well, I'm thinking of this as a combined Simulation - Adventure game. Ie. You can do these things - at any time, as opposed to being tied down so that you can only mop the floor when it is "bloody". Or buy flowers if you've upset someone, if you buy flowers when you haven't nthey'll either be pleased or suspicious that something is going on.

Think of it as a general purpose cause and effect simulation, what you do can be done at anytime, thus avoiding the irritation of a game saying You Can't Do That Yet.

And each situation can have different results, not so much solutions as repercussions of your actions.

That would be the ideal sandbox, but what sort of rewards will you offer players? How will they know when they're doing well? What sort of conflict or competition will be available? If it's just a world where actions lead to reactions and every option is always available, you'll usher in a new age of nihilistic video games.

How will you make these free-form choices matter?