Sloth Bears are one of my favorites, but are they a CITES species? If so they may need the regions support in obtaining them.
Also the fact that Melbourne seem to be considering multiple species is a reflection of not only the amount of space the complex has, but how much of a whole the elephants would leave. It wouldn't be 'trail of the elephants' without elephants! So they'd need some sort of star in return. An orangutan expansion is an obvious form of making them the 'main attractions' of that precinct; and is probably the most likely as it's been suggested by people within Melbourne.
A tropical dome would be wonderful and if the Islands Idea is taken into consideration, it could potentially occupy one of the paddocks (probably the final, bull one). As
@PaddyRickMFZ said before, the Philippine Crocodile need an actual habitat where they're one of the star attractions. They would get such an exhibit if a Tropical Dome was built; and would most importantly, give them the space to breed.
Although the Japanese Island expansion is a wonderful idea, I have to admit it isn't very realistic. What stands is a potential expansion to the island opposite. Would that be enough? Potentially. But I'd argue Melbourne would be wanting to build something world class that can not only equal the elephants past glory but possibly even go beyond that. If they expanded that lake even more, and added another island (possibly where the first elephant enclosure is), that would create a nice lake area (with the trail following the outside of the lake in a sort of loop).
Komodo Dragons could then potentially receive an enclosure in the current Elephant Barn. With the other two paddocks then taken up by; a tropical dome and maybe even, Sun Bear (if not Sloth Bears)? The Binturong could then inhabit their former enclosure and that would be an area well done imo. They already have that Thai themed village which could also serve as a plaza if a cafe or something's introduced overlooking the lake and the islands. It's probably the best direction Melbourne could head in, and they wouldn't need to do much re. imports ect.