I'm fairly certain that it would be utterly impossible to maintain NPOV with this. Short of having commons:Category:Topics as the start point for settings, any system will skew the censoring options towards the needs some particular culture(s), putting lower priority on others.
Topic on Talk:Personal image filter
Yes! To say positively: Make commons:Category:Topics the starting point for these settings and more explicitly do not create a new category system (or the ugly terminology that must be associated with any not-just-descriptive second-category system). Any lesser solution would only feed controversy. Thank you!
Tacking the filtering system onto existing categories would make the Commons category system the target for endless reversion wars. A separate category tree can be protected against vandalism, wars are then carried out over individual images, but that would not affect the Commons project (an image can be added/removed ad-infinitum to the new category tree without affecting anything else) - as opposed to the extensive and rapid mayhem that could be wrought by fiddling with the Commons category structure to alter what was filtered.