The current image categories are functional, easy to use by the authors and buyers, and work great.

However, since Symbiostock was designed primarily for the benefit of the artists, it doesn't have to be as restricted as the conventional agencies. I envision the underlaying image database as a powerful and flexible platform being able to store additional information that could add even more value to the system.

Specifically, I'm thinking about additional categories. For lack of better word, I'd call it artist categories, type, groups, or something similar. Maybe even two more fields - type and group.
The users wouldn't necessarily see or use it, but the contributors could use various codes to assign more meanings to their images, and then filter on it and possibly even perform other processes on the filtered sets.
For example, we could have various pricing codes, filter on them, and then perform a bulk price change. Or a "original" and "derived (cropped) image" category. Perhaps creation year or maximum image size. Color workspace and other technical data, such as camera, lens, or filer used.
Everybody hase different needs, but by adding a few extra codes / fields to the image database, we could configure and use the images creatively to make our "digital assets" even more useful and productive. Something between Lightroom and inventory item database.

Having those extra codes doesn't mean that they have to be used by everyone. Some people would find it vary valuable, some may never use them, and others might warm up to them once their basic site is up and running.

I think that he is asking for "hidden" categories.

The users wouldn't necessarily see or use it, but the contributors could use various codes to assign more meanings to their images, and then filter on it and possibly even perform other processes on the filtered sets.

I *think* that would be easy enough to do - do not add them to the drop down categories in the header and hide the main category page(s)  I think the only time they might show up then is if someone does a search on your site and you do not have any results?

But won't the "hidden" categories still show up on the image pages themselves?

if implemented, these should be separate from the current categories, otherwise it will be a real mess for reporting, sorting, display, etc. would be better to design it as new field(s) similar to the promoted keywords field -- works like a keyword, but only used by routines that know it's there

once you have the field, the values would be completely up to each site's owner