Growing Wild is the perfect place for a South American precinct, but thinking realistically, this is unlikely to happen. Growing Wild is relatively new and serves a decent purpose of education, so I can see it staying around for at least another decade or so. Zoos Vic priorities conservation and education, so I think we can always expect some sort of kids dedicated precinct at all three zoos.
The current Malayan Tapir exhibit is a little small for a Bongo, but if they were wishing to combine that enclosure with the Peccary one next door, it could easily suit a bongo.
The current island could be extended outwards a little to give the orangutans more space, but I don't think such expansion would give them much more space than without one. The moat can only extend a certain bit further before it'll be too close to the paths.
If Melbourne want to have a program similar to Perth's with multiple breeding females, a fourth enclosure will be needed. This is really where the decision lies. Do they want to breed going forward? If so, a new enclosure will be required, and if they want to have more than a single pair, two additional exhibits will be required.