Not sure this is what you mean, but I would not implement that at the tag level (each tag marked as major or minor), but at the game level. So each game has a "Major Tags" and a "Minor Tags" but the tags themselves stay the same.
I wonder how having the mods manage the Major Tags, and having a wiki style for the minor tags would go. So they only really have to manage the main themes but you can search based on the most common "I saw this in here" tags users put in. So instead of Major/Minor, it is Mod and User tags. I suspect this works better in my head than in practice though.
Could sure do with a Male Submission/Female Submission/Futa Submission set of tags though, as well as a few more specific fetish ones.