I have just few fonts installed. But I need to create some graphics and I need some fonts. I have idea how it should look like. But "yum search font" just give me names of fonts (hundreds of them, with no idea what is look like) and gnome-font-viewer shows me just those fonts, which I have installed.

It would be great if gnome-font-viewer can show me (in different tab) what fonts are available in Fedora and in which package they reside.

There is no need to reflect 100% reality. To update this list once per release should be enough for everybody.

I can imagine that you put in BuildRequires all available fonts, render those "Aa" and "quick brown" images and store it as bitmap image and in that tab "Available fonts" you will not render them, but display those pregenerated images, with information from which package this font comes.