I would love to see file name manipulation, like (batch) renaming songs, but still keeping the link between the file and the library.
Futhermore, I sometimes have duplicate entires in my library, because I got the file twice or I moved the file and therefore I got one entry, where the file is missing but has the right meta data, and another file that is not missing but has also meta data. Merging songs or duplicates and chosing what meta data and files you want to keep per track (with its duplicates) may be very helpfu within the library list view with extensive libraries.