Crossed three off my list today.

Good reminder about European primate populations for me. I would add any of those to my own bucket list!
A St. Louis/Chicago/Milwauker trip would let you cross off two duikers, bonobo, pangolin and both gazelles.Probably top of the list for me would be seeing a tarsier. Unfortunately I'd likely need to go to Asia to ever achieve this one. More practical species I'd really like to see include any duiker species (I've yet to see any), musk ox, Speke's and Soemmerring's gazelles, white-bellied tree pangolin, Sulawesi crested macaque, bonobo, Javan gibbon, and drill. I would've had a lot of birds on the list, however this year I've been to multiple of the big bird zoos and so got a lot of lifers in that category, and also got a substantial number of reptile lifers as well. For amphibians, I missed the Japanese giant salamander at Detroit and so that's a species I'd love to see at some point. If I ever get across the ocean, I would like to see some of the species kept in European zoos but not US zoos, such as silvery marmosets, white-fronted lemurs, crowned sifaka, talapoins, barbary macaques, pig-tailed macaques, king colobus, and chamois.
Good reminder about European primate populations for me. I would add any of those to my own bucket list!