What could I do with the alpha component of my frame buffer?

I currently don't use the alpha component of my f16 frame buffer. (The fog no longer needs it)
Using it would probably cost nothing on most hardware, so I could theoretically put some feature in there.

Also the blending for the alpha component is separate, so it can effectively be treated like an extra framebuffer.

Now sadly I have no idea what I could do with that :​(