In the shop I always figure no job is *really* a good one unless I have to get a new tool. In that vein, if a good cratered effect is Really Really important, consider getting Pro Fantasy's Fractal Terrains. It'll do single impact craters or wholesale bombardments. Not that I've ever generated a world then mercilessly pummeled it from above, no. Well, not many.

If your cratered wasteland needs Truly Huge impacts, keep the size of the overall orb small, since I think the biggest crater it'll do is 1000 units (you can use miles or km as your default measurement system). That way the proportional size goes up. Though really if you want mares and plains, you'd do better to start with a low-roughness world and/or do some manual tweaking.