Lots to unpack here. I messaged the zoo about the Gibbons and siamangs right away. According to the people who responded they would like to mix animals eventually but It is all up to space and what the keepers feel is right. Honestly I have know problem with the Gibbons mixed in. San Diego, is a pro the siamangs have even been spotted rearing and playing with the baby orangutan. And the Orangs have been helping the siamangs out by rearing the baby siamangs.
I am going to go off topic. Assiniboine I believe has a breeding pair of siamangs. They are huge fans of Hudson and Humphrey. I think you can guess where I am going. Just a simple win win trade. But that's just foolish and dreaming.
The empty exhibit will pose an issue. We all know how empty exhibits have an impact on guests. The Elephants. Old Styled enclosure, with few to one animals in it. We've all been through this before. But this time there is an improved space. But the image of empty exhibits is bad.
A few animals that can be mixed in with orangutans according to other zoos. babirusas, they are not super hostile and just need a wallow built since the outdoor exhibit will feature a stream running through it. Gibbons and other small primates can also work. Then Asian Small clawed otters, cute and cuddly, but keep the orang infants away.
In the end the zoo has oppurtunity but is not capitalizing on it.