Add ability to cancel a tag source lookup.
Ability not to save artwork to tag, or to tag & file or just file or not at all. 
The only big things I can think about are:
Rhythmbox and mpd library support.
Library stuff, like finding duplicates, maybe some stats (I hate that most
    programs don't provide good lib stats)
A better console version. It shouldn't do much besides editing tags,
running functions/actions.
Integrating the logging thing with the whole app and having logs for
most everything.
Massive, massive code cleanup.
Docs for everything that needs it like plugins and all the new stuff.