Anyone who has flown out of Melbourne and Sydney knows how bad Melbourne is in terms of procedures and spacing. Melbourne has much greater available airspace, but has a ridiculous set of STARs and SIDs that over complicate and hamstring the controllers. Simple fixes like having Turbo-prop SIDs that turn much earlier allowing much less spacing between jets and slower traffic. In Sydney a TP is turning over houses at 500ft AGL, usually with a significant vector that gets them out of the way of following jets, so a jet can pretty much roll as the TP lifts off. In Melbourne they have to climb to 1000ft AGL, and then usually get a vector that keeps them in the way of jet traffic for a full extra departure slot, mainly because they don't want to vector a TP over some houses. Then there's simple things like Sydney will line up and depart trafffic with half the distance on final Sydney will. Which means in general they get almost two aircraft away in a gap Melbourne can.
In short Melbourne could greatly increase the rate of departures by having more adaptive procedures for mixed traffic, and letting low noise traffic like turbo-props depart over noise sensitive areas, to get out of the way of faster traffic. Then implement Jet and Turboprop holding points so that you can pop the right aircraft into each gap, ie Jet then TP up its backside, or TP with large vector and jet up its backside, etc etc....