Actually just monkeys in general. Apes are well covered (we need gibbons of course), lemurs I would like to see more of, but 2 species feels fair.
But monkeys really lack diversity in the game, 3 species doesn't cover it.
Occasionally I see people request smaller South American species as well as baboons and barbary macaques, but beside from that people doesn't seem to care about them.
It almost seem like a big part of the community just doesn't like primates.
Monkeys are important zoo animals. They are very entertaining and active animals to watch. Their enclosures are often easy to fit in between larger habitats. They bring life above ground level, and many species work well in tropical domes and similar.