The Okapi and Colobus combined exhibit could work however okapis are very shy and allusive so have colobus monkey swinging around might not be ideal
My proposal
Cotton Top Tamarins remain were they are.
White cheeked gibbons move to the Japanese garden, if not the enclosure is being expanded
The colobus monkey would look great in the ruffed lemur but Melbourne probably won't to that. The monkeys could go into a interconnected exhibit current and the white cheeked gibbon or current and an overhead path to the emperor tamarin this would be a proud pleaser as it would be cool to have the monkeys above. The empire tamarins could be moved to a sampler exhibit as the are quit small. If the small end exhibit was extended a bit they could go in there
Gorilla Rainforest
Gorillas remain with a couple of female imports
Pygmy Hippos remain and import a female
Cassowary could go replaced with Mandrills or Okapi
Tree top Apes, Cotton top tamarins Remain, Spider monkeys remain, Emperor tamarins transferred to smaller exhibit
Colobus walkover exhibit Current and tamarin and the white cheeked gibbons expanded
Lemurs remain
You can Barley see the colobus monkeys as Jambo said. She is shy but the exhibit is very bare so some foliage were she could hide would be could
Trail of the Elephants
Otter remain in current exhibit same as tiger.
As Elephants are moving to a great new complex at Werribee. A sensible replacement would be the Indian Rhinoceros in biggest exhibit Blackbuck could also be included. It would be great to have the first exhibit replanned into a more jungle and have Malayan tapirs. This would be great but it is kinda unlikely.
The siamangs could stay the orangutans expanded into the islands in the middle of the water similar to the zoos in new Zealand. The squirrel monkey could have a better exhibit but they can remain.
Main trail
Red Pandas remain
African area
A new themed area
Giraffes and zebras stay. Nyala were supposed to be coming but didn't. They could add ostrich.
Tapir and Peccarie are turned into a nocturnal animals of Africa. Fennec Fox and Cape Porcupine Bat eared fox would be cool.
Lion Gorge
Dingos seem to be staying as wild dog were conferred as a phase out.
Lions Stay
Snow leopards stay
Coati will probably be phased out but could easy more be imported
Indrah could be replaced with sun bear with little work to the exhibit
Tasmain devilas moved to Australia with an alliance made with Komodo's national park 2 dragons imported
The only real changes I see happening within the next five years, revolve around the Treetop monkeys precinct, trail of the elephants, the Australian area and the Tapir and Peccary enclosures.
Treetop Monkeys
There's really no need for much change here, although I would be a fan of moving the Colobus, I do concede that Melbourne would be unlikely to do so. So we're being realistic here in saying this isn't a
likely option. I imagine they'd get the other former Capuchin enclosure too in addition to their current one, to give them more space. I'm not a big fan of the tinted windows, but an easy way they could achieve giving visitors at least
decent viewing would be to give a small viewing area towards these enclosures behind the Pygmy Hippos. There's currently a bamboo walk that can give you somewhat of a view of the Colobus monkeys, but is quite packed with bamboo so is hard to get the 'best' view. Clear some of this and give a viewing deck or something and it would be a compromise for the tinted windows above.
Elephant Replacement
The best option here would be to extend the Orangutan's space this way, and give them at least another enclosure. Alongside them, Melbourne could either import Sloth Bears and give them two enclosures. Komodo Dragons are another feasible option, alongside the Orangutans. Another option that shouldn't be overlooked too - Okapi. Although I'm aware Okapi don't fit the precinct; they could have their own mini trail off the main Asian trail - 'Secrets of the Okapi' or something of the sort, which could potentially feature Colobus monkeys too.
Australian Area
The first area to improve would be the Great Flight Aviary, and the best idea is to first, restock the bird collection, and second, move Koalas, Tree Kangaroos and Echidnas into the aviary. Koalas and Echidnas could share the first two halves of the aviary (Woodlands and wetlands) with Tree Kangaroos in the final rainforest section.
Echidnas could also move into the former Koala enclosure.
Top of the Zoo
With the Peccaries and Semangka both in their final years, those two enclosures will soon be empty. My idea would be to totally demolish both enclosures, and create an extension of the African area with smaller African species - Fennec Fox, Meerkat, Crested Porcupine and Leopard Tortoise. Three large rotational exhibits could be created for these species (with the Meerkats and Crested Porcupines sharing an enclosure). There's certainly the space for this; the Tapir exhibit goes a decent bit back.