My thoughts on sandboxing
I think Andreas is certainly on the right track. These are my comments which I decided to cross-link to the (LRPGSW) from his comments box, since my comment started getting a bit involved. I decided to Cross-Post this to the ElthosRPG Blog as well, because, obviously, different communities of readers may find it in either location.
I've run sandbox games all along so I'm pretty much sure that with what you have here would provide a good gaming experience for, as you say, pro-active players with some imagination. That said, I would add what I think is an essential concept to sandbox...
Create Pearls. Pearls are loosely coupled objects within your World that can be moved around and connected at will. Pearls can be people, places, things, or events. The pearls can be rolled out during the game when the NPCs or PCs trigger something that calls for one or more. What you probably want to avoid in your pearls are things that define them too rigidly within the world, although you can create strings of pearls that have defined links, for example an object-pearl that triggers another event-pearl that triggers a place-pearl. An example of a few loosely coupled pearls might be:
1) Lady Elaina, Princess of the Moon-Wolves Tribe.
2) Rathan Mountain, a jagged ice capped granite mountain on which is an ancient oak forest in which wolves and dryads live in uneasy co-existance.
3) Chaknorak, Wolf-Bane Sword, +2 vs Lycanthropes, and will return lycanthrops to human form on critical hits.
4) The Rising of Elehan: An ancient coffin in which is the skeleton of Elehan is opened, and the corpse of that terrible Wolf-King reanimated. His first goal is to find Chaknorak, and destroy it, for that is the weapon that killed him long ago.
Now you can see the obvious link between these is the moon-wolf theme. However, notice that there are no dates listed. No specific location relationships, or direct connections. Only implied ones. Those three would make a nice string of pearls, but they need not be strung together. Stringing them would simply be a matter of adding a notation that links them more directly. For example adding a trigger to Chaknorak as follows:
3) Chaknorak, Wolf-Bane Sword ... The relic is hidden in a large cavern guarded by the phantom of King Elehan. Whomever touches the sword will awaken the King's spirit and be lead by hook or crook to his tomb, one way or another, over the course of several days at most. The sword will hum in the presence of the tomb, and this will trigger the awakening of Elehan, who will try to remove the lid of the coffin, but be unable to do so on his own since the sacred seal must be broken from the outside. Once open the ancient King will speak in a long lost dialect, and attempt to take and destroy Chaknorak. If he succeeds he will return to Rathan Mountain and seek to find his old throne, resume his Lycanthropic form, and revive his kingdom.
As you can see the adding of the trigger to pearl 3 certainly ties it much more closely to a timeline, place and events that follow.
Pearls are a good conceptual model for Sandbox games. You want to create a bunch of people, places, things and events that are loosely coupled (by theme perhaps), but not tied together too rigidly. This gives the GM a lot of fun things to throw into the game, and yet not get tied up by criss-crossing complications due to rigidly defined timelines or relationships.