This could reuse some existing mechanics used for placing bricks and concrete floors and allow for the creation of custom elevated gardens and similar, but in regular mode (vs sandbox). It could also be included as a task in Jobs.

Let's say the player makes a box out of concrete floor and brick walls and they want to fill it with soil for a rooftop garden, they use the 'fill soil' tool (could be listed with lawnmower and snowblower) and select the area they want to fill similarly to concrete floors (basically swap out the model/texture).

The area then fills with a soil layer. Like placing bricks, clicking and holding again adds another layer on top allowing deeper soil pockets.

Player builds have tried workarounds such as various 'raw' materials that look vaguely soil-like but it's not ideal.