I know this has been discussed and I am glad planet zoo is still being worked on, I really hope they add the two most needed and missing DLC soon as this is the most important parts of the zoo the we need to create.
Aquariums are definitely not an important part of zoos. It's pretty rare for zoos to have extensive aquariums in them, and even when they do it's usually smaller animals such as coral reef fish.
For an aviary one, I'd like to see something like a larger exhibit as an option for several of the smaller birds, so you could have things like several older zoos have full of exotic songbirds
I really hope they add aviaries and aquariums, doing so would open so many possibilities. Exhibits, off topic, need an overhaul as well. They feel very restricted and changing this I believe would be a first step closer to aviaries and aquarium.