In Airbus narrow body family the a/c have three letters and a new a/c of the same family have gotten three other letters, A318/A319/A320/A321. But in the widebody family other derevatives got a sigen after, i.e. A340-200/-300/-500/-600.
So I don't think that we will se an A339 ever.
Just like birdwatching - without having to be so damned quiet!