Picking up the Monkeys/Primate conversation, I do feel like given the massive amounts of species within, if we include apes, monkeys, lemurs, tamarins, etc, that we have a relatively small amount. Primates are not one of my favorite groups of animals, but they probably overall are the most needed out of all the mammal groups.
Ideally, at least 2 more New World Monkeys. Howler Monkey for sure and either the Spider Or Squirrel Monkey. Or all 3. But at least 2 of them.
I'd love see a tamarin or marmoset, but like
@DarthQuell mentioned, they're so small. I'm not sure how I feel about them in WEs, maybe would be okay.
Lemurs, I'm personally quite alright with the 3 we have, but I wouldn't be opposed with another. It would need to be visually distinct though.
Apes, I too think we're in a good spot, but I'm not opposed to and we'll probably get a second Gibbon.