There is plenty of room for quad 3500m runways in the 05/23 direction if you take the area between the river and existing Hawkesbury Valley way. Roughly plotted, a pair of runways either side of the existing RAAF Richmond Base. Thee will be some infill required to level the flood plane, maybe even a need to deviate the river so you can have round the end taxiways.
For a departure in the 23 direction you have 8nm before meeting the high (well what we call high- I'm sure the Europeans would laugh at us) terrain. RNP standard departures would easily fix that.
The technology is there, there is a massive need for landfill sites in Sydney. We just need the will.
Of course, it will never happen as we have become a nation that lost it's innovation in the 60's.