We have sometimes have situations where two parts are almost mirror images of each other, but not quite. One part will have a hole or two that its counterpart doesn't. This presents us with a choice:
1) Make a mirror image derived part and then add or remove features as needed.
2) Forget the derived part and model both parts from scratch.
I'd like to see the ability to generate a mirror image part that wasn't a dumb solid and had all the sketches, features, you name it, of the original part, but in mirror image. I would not expect the mirror-image part subsequently to update with changes in the original part. If I did change the original part and wanted the mirror image part to reflect the changes, I'd derive the mirror image part again if that seemed the most straightforward approach.
A couple of years ago I ran this idea past someone from our reseller at user group meeting, and he said I wasn't the first person to request it.