I think a big part of the reason why European zoos breed manatees and American zoos don't is that they keep different subspecies - in America, they keep native Florida manatees, while in Europe and Singapore they keep Antillean manatees. The Antillean manatee has a much wider range but are much rarer, and zoos are starting to send their captive-bred manatees back to be reintroduced. In 2016, the first two manatees were flown out from Singapore to Guadeloupe for eventual release. I would personally just choose the West Indian manatee (which combines both the Florida and Antillean subspecies) and make them breedable.
The other species I would choose are:
- Leopard shark (a fairly small climate tolerant shark that can be kept in open-air pools and mixed with penguins (San Diego Zoo has them in their outdoor penguin pool)).
- Macaroni penguin (a threatened penguin species that is similar in size to the African penguin and could live in a mixed display with the king penguin).
- Walrus (the third and largest of the game's pinnipeds that inhabits truly polar environments and would also be the game's only threatened pinniped species).
- Atlantic mudskipper (the game's first fish, which would be able to inhabit the same brackish exhibit as the diamondback terrapin).
I have chosen all of these because they would fit into existing themed areas or mix with currently-available species - I have never seen sea turtles not being kept in large aquariums with reef fish, for example.
