I guess anyone interested in the subject have seen Rob Conley's major series of posts on how to create a sandbox? I have a suggestion. Don't read it. Yet.
That series, while informative and exhaustive, is way too much. To start a sandbox you need far less. This module that ignited the fire fuelling the this train of thought provided far less.
- A sketchy map, with some atmospheric names, is great for inspiring wonder and creating ideas for adventure.
- Some NPCs who move about this area, with clear motives and plans for players getting involved in, helping or thwarting.
- Pro active players who can look at that map and run with the ideas generated from it.
- Player characters with ambitions and short and long term goals.
- A few McGuffins.
Note that I have never successfully ran a sandbox campaign, so what do I know?