We currently have greek, roman, and greco-roman tags. I'm not sure what to suggest as synonyms for what.
First, we all know that greek and roman mythology is so similar that "greco-roman" is a legitimate term in all academic circles, so perhaps greek and roman tags should be synonyms of a greco-roman tag.
But, there seems to be a convention developing where a good portion of tagging revolves around identifying the geographic location where a myth was prominent. For example, there is pacific-northwest, chinese, and irish, all in use to identify that the question is about myths from those areas.
Further, there are plenty of instances where a question should have either a greek or roman tag, but not the other. (Greek examples: 1 2 3 Roman examples: 1 2 3).
So it seems like having a greek tag and a roman tag is a necessity.
The question now: Should we have a greco-roman tag, or should questions just use both greek and roman as needed and greco-roman should be a synonym of both?
Similarly, norse and teutonic are near synonyms, but they are different. Reference a current meta post.