For something special, maybe a huge animal pack with a theme like 'safari' which is so broad it could include a lot of animals.
Or Petting Zoo themed, which would be something really different and includes domestic animals such as cattle, sheep, goats, donkeys, pigs, ponies, chickens, add new scenery, and also add new guest- animal interactions (like feeding, animals coming towards people instead of running away from them. Maybe these tame animals could even be allowed to free range, meaning they do not need a habitat and guests never run away from them. Poop is cleaned by caretakers and treated the same as trash.
Or ... Yes really... Something museum and extinct themed, bringing in scenery like skeletons and an archeology dig site for guests to explore, a new function called the genetics lab, and you can add a host of extinct animals to your zoo like tasmanian tigers, dodos, aurochs, great auks, mammoths, sabretooths, moas... Really awesome stuff. No dinosaurs, they should all be animals that people once lived with.
The last dlc is bound to be something big and cool. Ending the game with something special.