Since foobar2000 now has official Opus support it would be great to be able to tag opus files using Picard. (Thus getting rid of the ID3V2.3 vs. 2.4 issues)
Opus is open source and uses a tagging standard similar to vorbis/flac (AFAIU)
www.opus-codec.org
Mutagen will have support for OggOpus in 1.21, whenever that will be released: https://code.google.com/p/mutagen/issues/detail?id=115