Well I definitely could use this, actually I've always wanted such a feature in winamp. Also, thanks for coming to this forum to ask us lowly end-users for our opinions.
Since I'm sure you've thought through all this already, I'll be brief. These are just a few features I think would be important:
-Transfer over the playcount, ratings, etc when renaming a file that's already in the database. Also it would be nice if the date modified listed in the ML isn't changed, but I don't if that's possible.
-Choice of whether to read the data from the tags or from the ML. Probably ML would be preferable, since the ML is always correct (unless the tags were edited in an external program and the ML wasn't rescanned, but that's rare) while sometimes the tags may not be correct (like if the user edited metadata and didn't check "update file tags" or if there aren't tags like in the case of .wav files). Also, just using the ML saves you the work of supporting multiple tagging formats.
-If there are files in the same folder as something that's about to get moved, ask the user what to do. Give them a listing of the files and allow them to select, for each one, whether to leave it there, move it along with the media files, or trash it.
-OPTION TO EXCLUDE CERTAIN FILES OR FOLDERS from individual functions of the plugin. For example, I collect bootlegs, and the filenames are important and need to stay the way they were when I got the files. Usually they're named in the following manner: phish2004-02-18D3T1_vbr.mp3, which means Phish on Feb 18th, 2004, and track 1 of the third disc. The filenames tell you the best way to split up the concert onto audio cds, although the entire concert usually has the same album tag. For these files I'd like to tell the plugin to do everything but rename the file.
-To identify whether a file should be counted as a single track or part of an album, you could do a ML querry on the album and artist and count the results.
Hopefully that's enough to keep you busy