Sand box ...
If you can, investigate Procedurally Generated Content to try to 'fill your sandbox'
It doesnt have to be for every aspect of the game, but you need sufficient (regional)randomness to make for differences across yoiur world.
Understand that most of the PGC scripting involved will be constraints to limit things to logiical/cohesive options.
Consider this will also involve 'Hierarchical Templates' being used to minimize work (as much as is PRACTICAL)
Part of the 'game' would be observing and recognizing the elements in the current situation to knw how to handle them...
Having some distinctive visual features (even if just overall color themes) to differential one area (with its own flavors) from another might help to get rrid of the 'everything the same everywhere' feeling players have about many 'sandbox' enviroments. And with the 'template method, having the Hierarchy Templates have 'variables' for substitutions of many (local) detailing bits (props, color schemes, etc...) you can have the system create significant visual variances without having to Hand Craft it all)