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I don't think it's a case of whether they "could" or "should", but whether they will. I don't think they will, and I think thus far the patterns people have noticed when comparing Zoo to other Frontier games have proven mostly correct. For example, the new content in the winter, spring, and now summer, and the themes of the first three DLC packs.
I think the future of Zoo will go one of two ways - either more substantial content compared to what we already have, a la the PlanCo Spooky Pack or the JWE packs like Return to Jurassic Park or Secrets of Dr Wu, or less substantial, more focussed content like the PlanCo ride-only packs or the JWE dinosaur packs. In the former case we might see things like birds or semi-aquatic creatures added provided such things are possible with the game engine, where the new content is supplemented with a new gameplay experience (aviaries, underwater environments, things like that which provide a new challenge and a new way to be creative beyond "new building pieces"), and in the latter case we would simply see new animals being added.
Also, I'm not saying this to be deliberately negative - as with probably everyone I would absolutely love it if we could have some kind of second version of the game like you suggested in your "2.0." thread, with a whole host of new animals, scenery, and gameplay options, but I try and manage my expectations considerably.