A Planet Aquarium may work, as there's been aquarium games before and there are some non-zoo aquariums. Still, it would be much preferred to have aquatic species in a zoo game. Even Prehistoric Kingdom, JWE and the old Zoo Tycoon games already had this, so having a separate aquarium game would make little sense. Planet Aviary would be even weirder as there are very few zoos that have only aviaries and no other exhibits. Aviaries, if implemented, should come in a zoo game and aquariums preferably as well.
If we don't get fully aquatic or flying species in PZ, I think there is a very good chance they save this for PZ2 which will most likely come some day. It may be the one defining feature that distinguishes both versions of the game, and it opens possibilities for new DLC packs after the most popular terrestrial animals are already in the game (assuming all animals from PZ will be in the main game of PZ2). Still, I would much prefer to have an update for PZ which introduces both features, which means it should come at the end of this year. If not, I don't expect to get any of these.