I’m inclined to believe that the move is just to give the giraffes more space (without the risk of being occasionally separated from the zebra), or potentially to hold a medium sized group of Nyala like @Zoofan15 said. They would be much more attractive and engaging compared to a small group of zebra and would potentially also be more compatible with the giraffe.
Bongos probably won’t return to Melbourne. Werribee would be my bet if they were ever to return to Zoos Vic but further breeding and imports would be required for that.
I’ve long favoured the housing of Eastern bongo in large herds and I think the history of this species in the region has proved pairs aren’t ideal for optimal breeding and succession planning.
By housing them at Werribee, this would at least allow them to establish a decent sized herd. There’d also be space for off display yards to accomodate surplus bulls to sub in for the founder bull in the breeding herd as and when required.
It’s hard to say whether Okapi will eventuate. I’m hopeful Taronga’s apparent plans to build the gorilla exhibits first is as much around delaying the Okapi exhibits to coincide with the eventual Giraffidae IRA as it is around prioritising vacating the current gorilla exhibit to utilise that space for the precinct.