They could - stress could - take a leaf out of the Japanese book and build a new runway on an artificial island in Botany Bay pointing out through the entrance to Botany Bay.
This would allow them to:
(a) use the existing transport system and 90% of the airfield infrastructure of KSA whilst giving them enough space to build new world class passenger terminals,
(b) be far enough removed from built up areas immediately to the north to allow 24 hour a day operation, (yeah, right!)
(c) probably end up considerably cheaper than any other option, and
(d) not suffer quite as many of the inevitable noise complaints and demands for curfews ANY other option will certainly attract within 12 months of the airport being built, wherever it will be built.
Unfortunately, I have it on good authority that any such artificial island would interfere with the annual migratory path from Kurnell to Brighton of the rare as bat**** Botany Bay three-toed sulphur crested perinewt, so it'll never happen.