Found this neat 2015 blog post by Alex Schroeder sharing what amounts to his sandbox game "style pitch" to players.
It's already a pretty short read, but Alex further condenses it into the following two-paragraph explainer in the comment section:
We’re playing in a sandbox. Dangers are not adapted to the strength of the party. Generally speaking, it’s safer near civilized settlements. The further you move into the wilderness, the more dangerous it is. That’s how players control the risks they want to take.
You learn of rumors from travelers in taverns, merchants at markets, sailors at harbors, books in libraries, or sages in their ivory towers. This information is not always accurate or complete. Use these rumors to add new locations, goals, and quests to your map. The actions of your characters determine the direction the campaign will take. There is no planned ending for the campaign. As long as you keep investigating rumors, exploring locations, and following quests, I will keep developing the game world in that direction. The harder you look, the more there is to see.
Super straightforward and contains nothing at all surprising, but I figure it's a good starting point for any genre- or setting-specific tweaks you might want at your own table.
