Hi there!
I've been using your program for a long time and love it, but I really think it could use flac support.
If this has already been discussed I apologize, I couldn't find it.
Thanks again!
Do you mean something special with "FLAC-Support"?
As long as I know MP3Tag it can deal with FLAC.
Huh???? Almost all my use of mp3tag is with FLAC files.
The same here. I started using FLAC almost exclusively for everything.
The only time I use MP3 is when I export (and convert on the fly) my library files to my portable MP3 player, using MusicBee (another wonderful piece of software!).
We can use FLAC/Vorbis tags without any limitation using Mp3Tag.
The only sad thing is that sometimes we have to do some mappings (manually) for things to work properly.
I use a few mappings that could be useful to others:
Tag Source Target
VorbisComment DATE Year
VorbisComment ORGANIZATION Publisher
VorbisComment TRACKNUMBER Track
VorbisComment SOURCE MEDIUM MediaType (source for the digital files: CD, FLAC, MP3, etc)
VorbisComment ORIGIN WEBSITE WWWAudioSource (Web site: MusicBrainz, Lat.fm, Discogs, etc)
VorbisComment INITIAL KEY InitialKey (only for classical Music)
You can check these two links with the real mappings Mp3Tag and MusicBee are using:
Mp3Tag: https://docs.mp3tag.de/mapping
MusicBee: http://musicbee.wikia.com/wiki/Tagging
That will help you decide what to do. 
As for standards:
Vorbis/FLAC:
https://wiki.xiph.org/VorbisComment
ID3v2 (used by Mp3, but also as base for others):
http://id3.org/id3v2.3.0