[lacker]

In general, if there is some functionality other web sites don't have, there is no icon that will make this new functionality clear to a user.

If clicking a button will make it zoom in, use the standard magnifying-glass-with-a-plus. If clicking a button will make it zoom out, use the standard magnifying-glass-with-a-minus. If clicking a button will open some other weird zoom mode then it is guaranteed not to be clear. Might as well just use a magnifying glass with no text then to imply it's zoom-related.

I wouldn't be surprised if the best design is just to find space for an extra button somehow, and use the 2-button-zoom that is so standard.