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What are examples of grammatical cases that are not attested in natural languages, but occur in some conlang?
To qualify here, the conlang should have at least some bit of flesh (e.g., a few consistent texts, either original or translated). Pure sketches of potential cases aren't interesting to me.
The Respective or Dedative case in Quenya, also called Short Locative, is too speculative to be counted here; its attestation is very weak in the available material from J. R. R. Tolkien.
I think the best conlang to look for such cases is Ithkuil. http://ithkuil.net/04_case.html contains a list of all cases. My [short] research suggests that some of them (e.g. navigative for "noun relative to whose vector, arc, or trajectory of motion an event takes place" or allapsive for "amount of time that expected to pass between the contextual present and the time of a future event") aren't attested in any other well-known language. I may be wrong, but it seems plausible regarding Ithkuil's goal of expressing extensive detail.
