We know https://www.wikidata.org/wiki/Lexeme:L502 is language: https://www.wikidata.org/wiki/Q9072
We know https://www.wikidata.org/wiki/Q9072 has https://www.wikidata.org/wiki/Property:P2989 to all the relevant cases
As such, it would be great to offer the user the list of relevant cases to choose from in Grammatical features for a form. If a case is missing, it just needs to be added with P2989 (where it should be anyway).
Similarly, it might make sense to create a property for grammatical numbers that exist in a language and leverage that in Grammatical features.