A second airport may well become necessary, and should be planned for, but in the meantime, how about dropping the curfew at YSSY? The earlier thread discussed in depth methods that would minimise the noise footprint.
The NSW Labor Government is in an almost unique position where from a political perspective, they could safely drop the curfew. Why 'safely'? - because whatever they do, they couldn't be any more unpopular with the electorate than they are already.
Richmond might be the least bad option, but, as has been stated above and on the eaerlier thread, adopting it will involve overcoming enormous operational and political hurdles. Anyone who doubts the operational problems need only take a look at Google Earth to see three of the most obvious - the townships of Windsor and Richmond located quite literally at each end of the existing (inadequate for civil long haul ops) runway and the railway line that in in the way of any north/south runway. And the fog. And the terrain falling away immediately to the north of the base.
As for the political implications... a veritable minefield because of the many MacMansion-packed dormitory suburbs surrounding the immediate area and the totally inadequate feeder roads to the airfield.
If the decision had been made 30 years ago, when it should have been, at least they would have avoided the problem of the inevitable - and understandable - demands for a curfew from the many residents of the immediate area.
99% of the residents around the existing airport on Botany Bay have no such excuse. They moved there after the airport was established and many of them, when the aircraft using it were considerably noisier than today's high bypass jets.