autoflight,
Each airport/runway has its own characteristics and aircraft types. Brisbane is currently single runway, but a parallel is getting there. That new runway will support simultaneous ARR and DEP operations.
Brisbane has quite a bit of heavy traffic. This makes it a bit more of a challenge for closer spacing options. The high speed exits currently in place (DC-9 era) offer little relief.
Environmentally, getting a new track approved is a very big challenge.
One scenario that may help at Brisbane is a batched diverse DEP. The DEP procedure can really only be optimized over the bay, so it would be weather dependent, but going to 400/500 feet and turning left and right, combined with a batched departure, would add a few more slots...IF environment would approve.