It would be convenient if wildcards could be used within the 'Remove Fields' and 'Remove Fields Except' actions.
Within a Remove Fields action, you could specify something like MUSICBRAINZ* to be rid of all MusicBrainz related fields, without needing to enumerate them all, or even know every single one. Another example: REPLAYGAIN* would remove all ReplayGain fields. I have an action group defined with a number of similar actions with related groups of tags in long semi-colon delimited strings.
(Here's an older request for the same thing, but I thought it better not to dig up a four year old thread.)