The West Metro to Parramatta has been announced as the preferred next metro line, with stations confirmed for Parramatta, Olympic Park, White Bay and Sydney CBD. Other stations are to be considered but have not been mentioned.
For the announced scope (ie: terminating in Parramatta), the biggest question in my opinion is how to fill up the announced 40K/hr capacity without hitting bottlenecks elsewhere in the "old" rail system. To see how big 40K is, have a look at the left bar chart below (source = TfNSW).
* In yr 2051, TfNSW forecasts 5000/hr station entries at Parramatta and 2000/hr transfers (eg. from bus, Parra light rail, Cumberland heavy rail, etc).
=> This is just 7K pax/hr potentially catching West metro at Parramatta station.
* A super-fast metro at Parramatta might induce more transfers from the Western line, but the Parramatta's existing four platforms (on the "old train lines") have only 25K/hr capacity. (Benchmark = Wynyard today, which also has four platforms and is congested today at 25K/hr). Note TfNSW are forecasting 19K exits at Parramatta.
=> Parramatta's potential latent transfer capacity is only an extra 6K/hr.
Max potential Metro boardings at Parramatta = 7K + 6K = 13K/hr.
For the announced intermediate "destination" stations (OP, White Bay) these will likely have exits > entries, ie: by the laws of arithmetic, train loading will drop below 13K at these stations. (eg. OP will have 20K jobs = 6K exits @ 30% rail mode share).
So how else can patronage on West Metro be boosted?
The answer is to add 1~2 further "interchange" stations, to feed the Metro. See right bar chart above for an example - Strathfield has 15K entries+transfers, which if they all were diverted onto the Metro, will double the potential utilisation, from 13K to something like a max of 28K (in practice more like 20K~25K as some pax will still catch "old" train lines). Furthermore, Strathfield has 8 platforms and hence has double the "old train line" interchange capacity of Parramatta, so it has much more headroom to induce even more transfers to Metro than the base case 2051 forecasts.