I'll go with Cairns Aquarium for this one:
1. Largetooth (Freshwater) Sawfish Pristis pristis
Kept in the past but outgrew the billabong exhibit in which it was kept. Would love to see the species return, perhaps it could be moved to the oceanarium once it grows too large for the billabong.
2. Any cuttlefish (Sepiidae)
Really any local cephalopod species could go here, but I think a large cuttlefish species like Ascarosepion latimanus would be a great addition to the barrier reef gallery. Nautilus spp. would also be great, but I think less likely to actually be considered.
3. More rainbowfishes (Melanotaeniidae)
Currently Cairns Aquarium displays two rainbowfish species, Melanotaenia splendida splendida and Melanotaenia sp. 'Malanda'. The former is kept in numerous tanks as a filler fish, so I think it would enhance the collection to replace some of these with other species - perhaps M. eachamensis, M. trifasciatus, Cairnsichthys rhombosomoides or Rhadinocentrus ornatus.
4. Terrestrial/freshwater arthropods
Formerly Cairns Aquarium had quite a nice collection of insects and arachnids in their rainforest gallery, including Golden Orbweaver, fishing spiders, diving beetles and tarantula. Unfortunately now most are long gone, and only the stick insects and Giant Burrowing Cockroaches remain. I think it would be great to see some of these species return, or new ones to take their place.
5. Flashlightfishes (Anomalopidae)
The nocturnal tank in the reef gallery, which houses a few species of squirrelfishes and soldierfishes, seems to be generally ignored by most guests. I think the addition of some more interesting nocturnal species to this tank, such as flashlightfishes, would help remedy the situation.