This is a great plugin. Whilst the "cropping" is a great feature (and much used), I would also like to support the ability to make the crop "optional" - as discussed above. So that the transformation can optionally apply to the entire image (which probably means the canvas would need to be enlarged in these cases).
Sometimes the convenient cues that you wish to use to straighten an image do not constitute the entire image.
The equivalent "perspective correction" tool in "Paint Shop Pro" (PSP) does not crop by default and instead applies the transformation to the entire image. In these cases I crop the image after having performed the transformation.
For example: (Screenshots from PSP)