Following on from image focal points, it should also be possible to set a "crop hint" on an image asset.
This would be a user-selectable, square area of the image. A derivative could then be configured to use crop hints, if present.
The image will then be cropped to make the crop hint area as large (and centred) as possible within the derivative's dimensions (without enlarging the image above its original size).
This feature will be especially useful for generating thumbnails, where you might want to focus on a specific area of the image which would be too small if the whole image were reduced in size.