Okay im back home with some time on my hands, so im gonna do a part 2 to my first few idears.
First i want to adress the fact, that i feel like it should not be planet aquarium. It can be named this way, but really it should be planet vivarium, with an overall focus on exhibit animals, as you could use similar mechaniks for both aquariums and vivariums.
First of all, i want 2 systems of habitat building, one like the one we have in planet zoo and one oriented on actual tank building. You can choose between multiple different sizes of vivarium/aquarium boxes (i mean atleast 15), which would open a second build menu, where only the tank is visible.
There you could design it like an actual tank, choose different kind of substrates to fill the ground with, multiple backdrops as well as the possibility to add your own billboard style and building pieces appropriate for the size, with each new size being able to use the items of the smaller one.
This would emulate like i said actual vivarium building, which is quite fun from my own experience. It has its one challagnes, upsides and downsides, but it is really, really fun and could add a ton of opportunitys. For the exhibit animals, i would include a traversable area system, that could determine where they are able to spawn, aswell as enrichtment style items, where they can spawn while performing special actions, like a foodbowl with a chomping beardie or a snake curled up around a twig.
With that in place, there should also be the opportunity to put exhibit animals into normally build habitats, with a nullgate being the must here.
There would also be some classic habitat animals i would add, that compliment the animals already present in Planet Zoo, meaning we can have some nice alternatives here!
Some picks would be the humboldt penguin, as they and the african penguin are about equal in captivity and it would be a nice pair to be able to choose from, gentoo pinguin, as a second and actually not that uncommon antarctic animals, thats also often paired with the king penguin, the african dwarf crocodile and the australian freshwater crocodile, as both are small and have the opposite in snouts and coloration, with the african havin a short and stubby snout and darker colors and the australian with a long snout and lighter colors, the asian small claw otter, because smallest, cutest and best suited for small tropical houses inside of an aquarium, agoutis, gundis and dwarf mongooses as small ground dwelling mammals and the common marmoset, prevost squirrel, crocodile monitor and emperor tamarin as aboreal animals.
We could obviously add more, but i feel like this is a good mix of smaller but still plausible habitat animals from all around the world and different biomes, that would make this game more interesting for people who want to just link it up as an expansion to planet zoo, aswell as just using the already given groundwork to give a little taste of planet zoo for the aquarium only owners. This would also spice up the gameplay loop, as building for terrestrial, aboreal and overall in comparison larger animals would give unique challanges and gameplay compared to the terrarium gameplay above and the bigger aquariums.
It also simply helps to catch planet zoo players that are unsure about the exhibit expansion, but atleast on a sale just cant resist the 12 habitat animals mentioned above.
Im quite sure there are quite a few people who would buy just the 12 animals for 20€, so the additional game would make the deal even sweeter.