Well, here's a screenshot of my plugin in its current (and very early state). I'm focussing on functionality at the moment, hence the UI is a bit cruddy - gotta love the Do other stuff button
This is the main (and incomplete) dialog where tags are fetched from MusicBrainz and applied to your music. The screenshot indicates how I spawned the dialog, through the right click context menu of selected music in my library.
Its first job is to split up the selected music into 'units', each of which should be matched with a MusicBrainz release, and displaying them as a tree. Currently it's doing this in a fairly naive manner by sorting by artist and then by album (or by directory if the album tag is missing). There are obvious potential flaws with this approach and I can alter this so that it divides the selection according to the current library view (i.e. Folder View, Artist, Artist/Album etc.)... or according to something completely different. How I should do this really depends on other things (such as the heuristic mentioned below), so I haven't pre-emptively implemented anything major yet.
When selecting one of these units, the current metadata is displayed - this works very much like the Foobar Properties screen and allows you to write new tags to the tracks.
Now I'm starting to work on the heuristic for matching the units with MusicBrainz releases. It occurs to me that I can use audio fingerprinting for popular releases and my own heuristic wherever fingerprinting fails. And if the plugin is unsure about which possible release to choose, it'll present the the user with the candidate releases (and affixed relevant data for each one), and the user will be able to pick one... ideally this will rarely happen.
This dialog is the crux of the plugin but there will of course be other dialogs, most of which will be in support of this one; e.g. a dialog which allows you to specify tag mappings for mapping the MusicBrainz data to track metadata.