In some languages, some complements of copulas can be in non-nominative cases, e.g. in Russian and Polish where they sometimes are in the instrumental case. A situation where such a thing could make sense in a language could be when there is some form of perceived gender disagreement between the complement and the subject, e.g. situations like 'she is a soldier', and this could make sense in a language even if the language lacks grammatical gender. However, I guess it would make most sense in a language with a grammatical gender system, whenever that gender system provides a mismatch.