Taxon-wise, factoring zoo presence & popularity, with the most important gaps bolded and underlined:
- New World Monkeys:
- Tamarins, Marmosets, and Night Monkeys:
- What we have: none
- Still Missing: Golden Lion Tamarin, Cotton-Top Tamarin, Emperor Tamarin, Common Marmoset
- Thoughts: This clade has over 2000 combined holdings on Zootierliste. I see them everywhere. They are adorable, marketable, reusable, popular, and often important conservation species. The golden lion tamarin is a top 10 animal, a famous conservation species, and featured on the Brazilian real. They would be an awesome addition. The other 3 are also somewhat popular and very common in zoos. The golden lion tamarin is my personal all-time favorite zoo animal, but even more tamarins would be wonderful.
- Spider, Howler, and Woolly Monkeys:
- What we have: none
- Still Missing: Geoffroy's Spider Monkey, Black Howler Monkey, Red Howler Monkey, Black-Headed Spider Monkey
- Thoughts: Spider monkeys are neat and common in zoos. They could use the brachiating frames, which would be exciting. We don't have a South American primate that can use them, yet
- Capuchins and Squirrel Monkeys:
- What we have: White-Faced Capuchin
- Still Missing: Squirrel Monkey, Tufted Capuchin
- Thoughts: This clade is common in zoos, and a representative from the two missing genera seems important. Squirrel monkeys are especially popular.
- Titis, Sakis, and Ukaris
- What we have: White-Faced Saki
- Still Missing: Uakari, Coppery Titi
- Thoughts: I really like uakari's as oddballs, and titis are cute and somewhat common in zoos. My dream primate roster would have both.
- Old World Monkeys:
- Langurs:
- What we have: Proboscis Monkey
- Still Missing: Colobus (probably Guereza), 'Classic' Langur (probably Hanuman, François', or Javan), Golden Snub-Nosed Monkey, Red-Shanked Douc
- Thoughts: The one we have is a bit of a meme animal, but it is an oddball and rare in zoos. Colobus monkeys are very popular and common in zoos. I really like them. They would also add a rep for a subfamily not yet covered, as would one of the 'classic' langurs. Those listed are all popular and good choices. Golden Snub-Nosed Monkeys are popular and super important to the massive Chinese playerbase, so they should probably be included. Doucs represent a third genus of odd-nosed langurs, and the red-shanked douc is somewhat popular and my favorite Old World monkey.
- Guenons and Relatives:
- What we have: none
- Still Missing: De Brazza's Guenon, Patas Monkey, Grivet or Vervet Monkey, Diana or Roloway Monkey, Schmidt's Red-Tailed Monkey, Wolf's Mona Monkey
- Thoughts: I've seen many guenons in zoos, especially the most popular De Brazza's guenon. Guenons are a bit of a weird taxon gap in the primate roster. The others would be nice and are somewhat popular, except for the last two, which are personal favorites. My dream primate roster would have all of them.
- Macaques/Baboons/Mangabeys:
- What we have: Hamadryas Baboon, Mandrill, Japanese Macaque, Lion-Tailed Macaque?
- Still Missing: Barbary Macaque, Gelada, Mangabey?
- Thoughts: I'm mostly fine with the current representation for this clade. However, the Barbary macaque is Europe's only monkey, and the gelada is somewhat notable for its popularity and presence in ZT2. They are pretty strong options. Mangabeys are a bit more unique, but they aren't popular.
- Apes:
- What we have: Orangutan, Chimp, Gorilla, Bonobo, Lar Gibbon, Siamang
- Thoughts: This is plenty for most people. Few want more gibbons.
- Lemurs and Oddballs:
- Lemurs:
- What we have: Coqueral's Sifaka, Ring-Tailed Lemur, Red-Ruffed Lemur, Black and White Ruffed Lemur
- Still Missing: Aye-Aye
- Thoughts: Aye-ayes aren't as critical as other species, but they are famous, somewhat popular, and would be useful for nocturnal houses. My dream primate roster would include them and maybe crowned and diademed sifakas as well. Lemurs are cute, and I've seen many in zoos. Usually, I've seen the ones we already have in the game.
- Lorises and Tarsiers:
- What we have: none
- Thoughts: They are cool, and I'd love to see them in Planet Zoo, but they aren't super common in zoos. I don't know if they'd work well, and they probably wouldn't be easy to make. I don't consider them critical for PZ, but my dream primate roster would likely have one of each.
Bare Minimum:
- ~2 Tamarins, at least the Golden Lion Tamarin
- Spider and/or Howler Monkey
- Guereza and/or Guenon(s)
Those 3+ would give me enough monkeys to build realistic zoos, but more options for variety would be nice. Those don't cover the Barbary macaque and GSNM, which seem like important regional picks although they aren't as critical taxon-wise or for representing zoos I've attended.