The zoo is rather boring. All you do is buy animals, breed them and sell products at the venues.
Here are some ideas I think would add more excitement to the zoo:

1) Have animals perform and learn different tricks.
--There could be a new manual allowing each animal in each specific enclosure to learn tricks to entertain the visitors at the zoo. With each trick, each enclosure would gain popularity points that would also be used to level up in the zoo.

2) Have the players groom and clean each animal in their enclosure.
--kind of like pets, each animal we care for needs to be groomed and have their waste removed to keep a clean environment for said animal.
--We could use soap and water to bathe the animals each day and also add points and experience to level up at the zoo.
**only selling goods at buildings in the zoo to level up and to gain popularity points is boring! We need more activities to do at the zoo.

3) Gems and leveling up
--each animal added and breeded uses up too much gems. How about each time we level up at the zoo we recieve gems as a reward?

4) Zoo-specific coins used only at the zoo.
--What about coins used in the zoo to buy items needed to build and maintain the zoo? These coins could be earned each time we sell hot products at the zoo.