Nice idea.

we get the feeling we don't want to see the images that are created by the concept "ReverseBirthing".


Your idea takes care of the conundrum we saw with the creation of fandom tags. Create one, ten others will want tags for their fandom too. That's why we grouped the specific fandom tags apart in the OP.

Having a fairly dense set of "core" tags will help the search system. Perhaps such a system should also have a "Not" option on tags as well as includes. People will almost as often search for a specific tag as they will strive to avoid certain tags.
We think generalized genre tags like "Sci-fi", "fantasy" "anime" and such should remain, as they give the general theme. More specific tags like "star wars", "reverse Birthing" or "Lord of the Rings" are better put in the Community tags.

Occasionally going over community tags and either purging virtually unused ones is also a good idea. If a community tag gets [i]really[/i] popular, one could even consider moving it up to "official" status, but that should take a lot to happen so that the core system doesn't  get bloated.

Ideally, one should be able to search both by core tags and by community tags.