I would love to have a field that knows what version of encoder was used to create a file.
It would of course be un-editable.
Some examples: Lame 3.97, FHG, Xing, Lame 3.92
MediaTagger has it, but unfortunately that can't do batch conversion so I've moved over to Mp3tag.
This is the one thing missing and I'd love to see it implemented in the near future.
This is helpful because I occasionally go back and re-encode some of my files with a newer version of an encoder, and it is handy to know which files are encoded with the old version.