Is it even ON the wishlist? can you explain why somebody would want this vs the .net tuple class? regardless of source naming guidelines .. (having type arg a and paramter a is really needlessly complicated).. personally I think you have a over obsession with functional style code.
C# isn't classically functional, so trying to treat it so is really pretty abusive and it shows with how you'd need to use it.. if you want to go nuts on functional style, why not just pick a language that is based on functional foundations like F#?
Your efforts would be much more rewarding there.