1. American bison. A zoo near where I live used to keep them when I was a kid, but got rid of them a long time ago. It's not a species you see in many places, so for that reason I'd want to include them.
2. Leafcutter ants. I love seeing leafcutter ant set-ups where you can see them climbing along ropes, so they'd be an absolute must for my zoo.
3. Honey bees. This may sound like an odd one, but bees are one of the most important species in any land ecosystem, and I'd argue bee conservation is much more important to educate people about than, say, giant panda conservation. Plus my zoo could always collect the hive's honey and sell it in the gift shop.
4. Not one singular species per se, but I would definitely want to have a coral reef aquarium tank with real corals, sea anemones, and species such as clownfish, yellow tangs, azure damselfish and Pacific cleaner shrimps. It's the kind of thing I wish more standard zoos had.
5. African elephants. Sadly only a small handful of zoos in the UK keep elephants now, so I would definitely want to have them as a signature species.