"IDv2 only if IDv1 too small" is my favorite feature of mp3tag. I want to use ID3v1 for all my songs and ID3v2 or APE when ID3v1 is too small. My files are all MP3, AAC, or APE. I'd also like to see ID3v2 writing for other files besides MP3. I have many standalone devices that read AAC and ID3v1/2 but not APE.
Maybe it would be a good idea make be able to make different tagging profiles for different file types.