I've said it multiple times, but I would really, really, really like a Children's Zoo pack with dwarf goats, squirrel monkeys, muscovy ducks, and turkeys.
Dwarf goats are extremely common, extremely versatile, and would work great with the color morph system.
Squirrel monkeys are common in children's areas and would be a very popular pick given the community's desire for more primates.
Muscovy ducks come in a wide array of colormorphs and could be used for barnyard areas, ponds, and South America multi species exhibits.
Turkeys would give the Americas a second birds and be similarly useful for both barnyard and wildlife exhibits. Could be subbed out for guineafowl, swans, goose, or a different pheasant.
Ideally, the exhibit animal would be budgies, but bearded dragons would also be on theme and give another Australia option.
Add a merry-go-round in the update as a ride option for generating income / increasing guest happiness and a few more guest/animal interactions and you'd have one of the best dlcs to date.