Don't know if this is possible, nor if it has been suggested before, but I was thinking while doing a little tag cleanup about a possible tag relationship that is the opposite of an implication. This relation would disallow any tags that would be nonsensical to have in the same post. For example, the tag "pokemon_(species)" would counter-imply the tag "not_furry", or "anthro_focus" counter-implies "no_furry_focus". I would love to see it as a feature, but feel free to bring up any problems that could arise.