One stadium is just not going to cut it. Sydney is a trainwreck in terms of access, fans won't want to travel to a single central location (Homebush) and would generally prefer to stay close to their own backyard.
In terms of new stadiums, Liverpool is a priority imo. Building it will allow the Tigers to abandon both Leichhardt and Campbelltown as playing grounds. It would also be perfect for the Dogs and probably the Saints once Kogarah is no longer viable (capacity, age without significant upgrades). There would be in the range of 20-30 NRL games played there with the Dogs and Tigers playing up to 12 and the Dragons 6. Somewhere in the region of 30000 would be ideal.
Blacktown is the second priority for Parra and Penrith. At the moment, Penrith and Parra stadiums are adequate for their tenants but with anticipated rises in crowds will likely find themselves being inadequate. Obvious spot is Rooty Hill, 25000-30000 depending on how crowds in Parra and Penrith improve. Rooty Hill would obviously require a train station but its an ideal spot for drivers.
ANZ isn't suitable for most NRL games, but to have it sitting there is such a waste. Make it the 'event' stadium of Sydney with blockbuster games, expected to outsell their homeground capacity, held there. Goes without saying that Origin and select RL/RU/Soccer internationals would be played there and then push it as a stadium for concerts and similar. Would expect it to get the rectangular conversion in the long term (pending whatever deal the AFL has with them). Push for special event transport to ALL games held there as they will actually be 'events'.
SFS looks to be there to stay so the best we can do is to make it easier to access which is the intention of the light rail. The proposed 60 000 is too big - I'm happy with the current capacity but they need to fix the bloody roof.
Cronulla and Manly are outliers and would keep their own grounds as shared grounds are not feasible. Keep them around 20000, small grandstands and hills. Don't want to spend a lot of money on single tenant stadia, just make them comfortable.
So a loose timeline would be along the lines of:
Current - 9 stadiums (ANZ, SFS, Campbelltown, Penrith, Parra, Brooky, Shark Park, Leichhardt, Kogarah)
Stage 1 - build 30000 all seater at Liverpool. 7/8 Stadiums - ANZ, SFS, Liverpool, Penrith, Parra, Brooky, Shark Park, Kogarah (with an eventual move to Liverpool)
Stage 2 - build 25000-30000 all seater at Blacktown. 6/7 Stadiums - ANZ, SFS, Liverpool, Blacktown, Brooky, Shark Park, Kogarah (with an eventual move to Liverpool)
Between stages - 'Rectangularise' ANZ, upgrades to Shark Park and Brooky
End product - 6 stadiums
ANZ - 'Event Stadium' - 80000-90000
SFS - Roosters/Souths/Sydney FC/Waratahs - 40000
Liverpool - Tigers/Dogs/Dragons (6) - 30000
Blacktown - Eels/Panthers/Wanderers - 25000-30000
Brookvale - Sea-Eagles - 20000
Shark Park - Sharks - 20000
I've gone conservative on all capacities because a solid and realistic average crowd of 20000 looks shit in a 40000 seater, 10000 looks abysmal. With the luxury of ANZ being adequately large and central to most of Sydney, we can afford to have stadia on the smaller side, improving atmosphere and reducing the overall cost.
You'd imagine this kind of end product would take in the vicinity of 15-20 years to become a reality.