Some of these features could be easily implemented by a third party; generally those that don't depend on intricacies of the similarity comparison algorithm. I'd love to see the features, and I bet such a cool program would attract various coders to do some of the heavy lifting for you!
- Multiselect (Shift-click + Ctrl-click to select)
- Customizable preference filters with good defaults (based on file format, bitrate, file location regex, heuristic recognition files that are part of an album vs. singles, tagged rating or other)
- auto cleanup based on heuristic defaults
- correct VBR support (if needed, simply based on actual observed file size divided by length, as a workaround if real support is complex)
- ogg vorbis support (and other formats)
- When deleting one file of a pair, support for keeping the "best" file, but perhaps also moving it to the "best" location (i.e. keep higher bitrate version but move to location where it is part of an album)