The reports I have seen (the Rocks!) seem totally unrealistic, but there may be waterside possibilities with CBD views, and of course THE bridge. But I think it is kite flying ATM, maybe some work being done in case the Melbourne event does self-destruct next year - not impossible given that Melbourne is opening up with massive numbers (by Australian standards) of new Covid cases compared to Sydney, they may have their sh!t together, but it may bite them on the bum too. In which case Sydney will say 'thanks very much'. It would be poetic justice after Melbourne stole it from Adelaide, a nice touch.
SMP/EC would be a goer, on the 'GP' layout, with lots of work. Bathurst would be a no-no, nice to dream about, but just 'no' in the real world of F1 safety. The other possibility is a brand new circuit somewhere in western/southwestern Sydney, close down SMP and sell it off, use the money to build elsewhere.
PI would be nice, but it is halfway to the South Pole as far as access is concerned - yes I know it draws lots of bikers for the MotoGP, but they probably won't turn up in numbers for the F1.
Oh, I have an idea! How about a street race in Adelaide ...