Yeah, I think one good example of core vs community tags would be to have a core "Fandom" tag that then can be fleshed out by some combination of community tags.

Seems to me that this idea of core vs community tags is a good one, it just predominantly seems to be a UI challenge rather than a conceptual one.

Finally, good point on pruning out unused tags. I also like a simple idea of requiring a mod to click Approve/Deny of every community tag before they actually become a selectable option, just to ward off basic abuse.