Useful

Improve code documentation
Option for auto-adding first level directories of pictures path to temporary target list
Photo ranges (start range, end range, Ctrl+T adds date span to target dir name)
Customizable FOTOSORT-TRASH location
Show image total and current
Image markers for fast jumping

Optimizations

Caching EXIF Data for faster loading, harmonizing image and EXIF load
Perhaps have pictures load in a separate thread for better responsiveness
Maybe keep the MRU path in conf and start loading from there

Nice to have features

Capture date/time correction
Auto-renaming pictures according to date/time
Rotating pictures
Zooming