How about a new option (-diff) for the list command that will compare the list of files (as currently produced by the -files option) with the list of files at a given local path (defaulting to the current path) and outputs those files existing locally but not in the snapshot(s)?
This could probably also achieved by scripting and I was thinking about giving it a try, but if duplicacy had this ability, it would be platform-neutral and more fail proof, I guess. If duplicacy would just output the diff-list, there would be still enough processing to be done by some script (like filtering for potentially important files that should be backed up, or so).