I use a bitmap for each character.

I have a custom made stream class (the streaming is delegated to a stringstream) that supports variable width fonts, various alignments, positioning, and changing the current color mid-stream. A font consists of an image containing all of the characters and a small file listing the widths of all the characters. A single character is drawn using texture mapping.

With SDL I wrote support for both truetype fonts and bitmap fonts (created with the Bitmap Font Builder) by following easy tutorials.
I'll probably use bitmap fonts only for the projects I start now.

I want to write a new filetype that can be used as tileset or for saving animations AND for bitmap fonts.
Not sure if I can do it though. But that is what I want to do in theory.
A file would probably hold several images and all the images different layers as sub-chunks.
So a font would hold all the letters/characters in its layers.
The font engine would have to know what to expect from the font resources of course.

Probably I'm dreaming though ...