Systems are interesting in sandbox games because they connect to each other. You can have a system that talks to another system, and then emergent gameplay what happens when the systems interact in interesting and novel ways. Re-usable, data-driven gameplay code is also cool, because it gives you a way to leverage work you’ve previously done on one system to make another system more powerful. Case in point: personal property.
One of the TODOs on my task list for awhile has been personal property. While you can build lower/middle/upper-class houses and beds, you can’t necessarily assign those beds to people, nor do people want a bed assigned to them. Instead, people just sleep wherever, whenever. This is going to change.