We're looking for a way to “tag” bugs & feature requests with certain product modules or areas so that we can better track duplicates and organize related issues together.

I don’t think adding more sub categories would really work since we're already using that for other purposes. To me this would be more of a sub-sub category, or a different field all together. 

Preferably it would allow multi-select. For example, I might want to flag a “Bug/Issue” as both related to “x module” and “y module” within a given product.