GIMP Plugin Registry

I find that you can use Resynthesizer to uncrop. That is, synthesize a ring around an image that pretty well matches the former edge of the image. In other words, widens the field of view synthetically. Works best if the edge of the image is stochastic, not patterned, like most snapshots would be that have a natural scene in the background.

Is that useful, or has it already been done?

I understand that if you crop, then undo, its much better. This is different, where you have lost what you formerly cropped, or if you never cropped but want to widen the field of view.

Probably LqR could do the same thing. LqR can do the opposite effect (loosely), where you can zoom the perspective (enlarge the foreground while maintaining the background.)