The Rail Bus and Tram Union has released a report titled "Fixing The Network". It advocates for 3 "shovel ready" projects:
1) North West Rail Link - Epping to Rouse Hill
2) Epping to Parramatta link via Carlingford
3) South West Rail Link - Glenfield to Leppington
They also advocate "Rail Strengthening" which is about upgrading signals and other infrastructure.
You can read the report here: http://www.rtbu-nsw.asn.au/multiattachments/4411/DocumentName/Fixing_the_Network_Report.pdf