Minifigures could be;
Zoo keepers - 1 in each Enclosure set.
Suited Mascots - in larger sets (Lizard Suit and Gorilla Suit Guy).
Random City visitors - 2 in each set.
Zoo employees - Janitor, Ice Cream salesman, Ticket seller.
The photograph below shows a prototype for an Ostrich Enclosure.
Other Set ideas could be;
1. Entrance with Meerkat Cafe - small entrance building with outdoor cafe and Meerkat enclosure. See picture 1 - Meerkat Cafe not included in prototype.
2. Butterfly House - Exotic coloured plants and Butterflies. - Added 22nd Aug 2013: http://www.flickr.com/photos/adventuresinlego/9561215699/in/photostream/ - See picture 1
3. Carnivore Carnival - Medium sized Lion enclosure with small seating area, with inside the enclosure: a Hoop and Stand for a Lion Show.
4. Giraffe Nursery - A Mother Giraffe has just given birth! Could come with exclusive Zoo Veterinarian minifigure.
5. Feeding Time - A Bear is fed yummy Fish by a small stream. See picture 5.
6. Camel Rides - A small >100 piece set, Minifigs include 2 Children, 1 Parent and 1 Camel caretaker. Each camel could have a specially designed saddle. Comes with Palm tree. See picture 4.
7. Gorilla Island - A Gorilla Mother and Daughter on a hill, surrounded by a moat of water. Could include a truck tyre for play.
8. Aquatica - See picture 4.
9. Elephant Pond - An Elephant swimming and playing with a ball while visitors take photos.
10. Reptile House - As shown in picture 2.
11. Ostrich Enclosure - See picture 3.
12. Zoofari Train: http://www.flickr.com/photos/adventuresinlego/9182361679/in/photostream/#
13. Zoo Kiosk: http://www.flickr.com/photos/adventuresinlego/9063554380/