Seems like a sharpening filter could be easily placed in GD's settings at:
For whatever reason, GD does not include a "sharpen" filter amongst all the other filters it provides...
... but there is a GD function called 'imageconvolution' which could sharpen an image for us. The function seems to need some not-so-simple parameters (e.g. a matrix) to work, but using some pre-built scripts we could make use of the function.
Up till now the only 'sharpen' type of feature I've seen in Drupal is the alternative to the GD module: Imagemagik. Imagemagik work's great, but some people aren't able to install Imagemagik on their server therefore can't access the extended settings for Imagemagik.
GD is built-in to PHP, so every body has access to it. The JPG compression setting being the sole option, thumbnails come out soft. Having sharpness control would be really cool. At least a single sharpness preset that kicks in on any scaling below 200px, that would help greatly.