This is a nice format, even if it feels wishful to some! I tried to make a balanced pack with freshwater and saltwater aquatic species (no birds, although I could find 1-2 here to swap out for pelicans/Inca terns, because I'm assuming we get a separate bird expansion if we are getting this aquatic expansion). I'm not a fan of having the cetaceans in the game, so none of those either. I tried to select species that can either go together in new kinds of mixed areas, or augment the major aquatic species and aquatic-friendly build-sets we have now.
First, a few much-wanted mammals to cover the waters of the cold north:
1. Walrus
2. Sea otter
Then a heavy-hitter from the Caribbean/Florida along with something to accompany alligators, bullfrogs, etc. in the American South:
3. West Indian manatee
4. Alligator snapping turtle
For me, the freshwater USP of such a pack is the ability to make good flooded forests with dwarf caiman, giant otter, etc. so a good roster for South America:
5. Pirarucu arapaima
6. Red-bellied piranha
7. Ocellate river stingray
8. Electric eel
Without a whole range of cichlids, mixed freshwater African tanks would feel incomplete, so I'm just going with a generic entry for Lake Malawi cichlids (with various kinds being the variants) and then two more small exhibit type species to flesh out African regions with hippos, monitor, the existing exhibit species, etc:
9. Lake Malawi cichlid
10. Mbu puffer
11. West African lungfish
I'm biased in favor of India in general, and I'd really like to give the gharial some badass Gangetic giant tank mates so one scary-looking man-eater of legend and one big gharial exhibit staple from the zoo world:
12. Goonch catfish
13. Indian giant narrow-headed softshell turtle
I'm saving Southeast Asia for the saltwater side of things, but one much-needed exhibit animal for East Asia:
14. Chinese giant salamander
Finally, Australia gets one very heavy-duty brackish/coastal species (that's also found in saltwater in other regions of the world) to finish out my freshwater roster with a critically endangered bang:
15. Largetooth sawfish
On the non-mammalian saltwater side, starting with a couple of big and interesting colder-water species from across the northern Pacific, potentially in areas with sea lions, sea otters, etc (specifically with one for kelp forests):
16. Giant Pacific octopus
17. Japanese spider crab
18. Giant sea bass
Then a whole lot of love for potential tropical Indo-Pacific/Southeast Asian/Australian coral reef tanks:
19. Humphead wrasse
20. Tasseled wobbegong
21. Ocellaris clownfish
22. Moorish idol
23. Red lionfish
24. Sand tiger shark
25. Green sea turtle
26. Leafy sea dragon
And then a few for the Atlantic/Caribbean equivalent (there's some overlap with the previous section, incl. the shark and turtle, so the idea is to just swap out the smaller tank mates):
27. Rainbow parrotfish
28. Atlantic blue tang
29. Green moray eel
30. Cownose ray
NOTE: I might have got some of the potential mixed-species combinations wrong, but am also thinking in terms of sections, so hopefully there shouldn't be too much that doesn't work.