Autocross layout editor is not a track editor, grass squares and dirt slabs are not needed.
Already existing car models disposed around tracks (ambulances, 4x4, truck, etc) would be nice and not hard to add however.
Actually slabs with grass and dirt surface wouldn't be hard to add either. We already have slabs that you can drive on thus ability to make/edit a track. Considering the amount of dirt tracks we have it wouldn't be such a bad idea. And if we also had curved slabs we could make some really proper rally cross layouts.
Car models on the other hand would probably serve more like a scenery props so why not add trees as well.