Birds, easily (and, IMO, obviously). In-zoo aquaria do occur but are not universal, but bird displays are. Extinct animals, fantasy animals, and cetaceans have no place in a game pushing a conservation and animal welfare message.
Even with aquaria I'd still only be happy with exhibit-style small aquariums which display a handful of marine creatures, such as a tropical fish tank or maybe the common octopus. I don't know how well that would work, though, since Frontier has said they want all animals to be 'individual' and you can't really display tropical fish outside of a community (and thus far, no mixed-species exhibits). I don't think such a feature is on the cards, though, otherwise we probably would have gotten something with the Aquatic Pack.
Birds, however, are certainly a realistic expectation at this point I think. It's the one major theme they haven't touched so far, even in terms of ground-birds (the only DLC bird we've had is the cassowary, which is heavily based on the ostrich model, so nothing really too unique). If they can figure out a good way to do aviaries then birds are the natural next step, and since they managed diving, after professing very early on that it was extremely difficult, they will doubtless figure out aviaries in some fashion, too.