I realize I'm not the biggest proponent of too many additional primates, but I do agree there are some that are needed and some that would be nice to include. And it's great there's a wealth to choose from. People think Primates and they just think Apes and Monkeys, but there's much more to it. even if those tend to take center stage in discussions.
Looking at the DLC, we have only gotten two monkeys, one of which is rarely held in zoos, and one ape. I'm okay with no more apes being added, but feel they will add another gibbon just to get an extra animal out of that rig.
Monkeys though are an entirely different story. I could personally give or take a baboon, but I realize it's fairly popularly requested; firmly in the second tier of requests if you consider the top tier being the top 10.
I think getting some new world monkeys is the biggest priority here; the howler monkey and the spider-monkey groups would be my suggestions, but also the tamarin and marmosets would be interesting additions. Though I do worry about their size being non-exhibit possible. I think the Golden Lion Tamarin and the Emperor Tamarin would be my favorites to add from that group, and I'm less picky on the marmosets. At minimum, and I stress minimum, 2 additional New World primates. But they could do an entire animal pack on that and still all have strong choices.
For Old World monkeys I already mentioned a baboon, probably the Hamadryas or Olive Baboon would be the most popular choices. In additional to that getting at least one additional Old World Monkey would be great, preferably one from Asia. I"m not really picky on which one but I think the Lion Tailed Macaque, the Golden Snub Nosed Monkey or one of the Langurs would be interesting additions.
Moving away to different types of primates, the Tarsier is another possibility, as is a loris, but I'm not going to be disappointed if they did not make it in.
Lemurs, I'm actually moslty okay with what we've got. . I can understand wanting the Sifaka, but with three lemurs already in game I personally feel like we've got that area of the animal kingdom represented. The Aye-Aye would be the exception to that, since it's a bit more visually distinct.