Tags that might be underused include things like "symmetry" and "rotational symmetry." The former tries to mirror and image and the latter does a rotational mirror like a yin-yang.

Pic related is a test showing off the effects.

It seems like these kind of tags would be unlikely for people to stumble across, you just already have to know they exist and what they do. Does anyone else know any tags like that that people might not know and would be hard to figure out?