Oftentimes an application is working with a file, and wants to attach a piece of information to a file: when this picture was taken, what class this essay is for, who photoshopped this, etc. Many, in not most, file formats have their own implementations for storing meta-information within the file contents: exif, id3, vim header lines..
I envision a library that can be used to list, insert and remove metadata from a file independantly of its format or storage method. The library would know of as many metadata formats as would be practical, and then would transparently handle the encoding/decoding process for the application. For formats like .txt, a pseudostandard could be created, hopefully as simple as json tacked onto the end. A call like meta_list_tuples(META*) would return -ERR_NO_FMT if there is no safe metadata standard for the specified file.
Note that there is already a library which implements this for (as far as I can remember) assorted image formats.. but I can't remember what it's called, and can't seem to find it again.