Here's a pretty solid idea to future-proof the aviation needs of Sydney-siders:
an underground rail line linking BK to the East Hills line at say Revesby extending to say up to Yennora station. This makes the link to Parramatta. BK airport (& the Business Estate) now become a very viable infrastructure and employment hub once again. If they're thinking a Macquarie Park syle future, they need door to door heavy rail.
Next, the new Badgerys airport has a line built connecting with Yennora. You can then land at any airport and easily access the rest of Sydney or depart another airport for onward journeys.
BK should serve as a commuter feeder for regional NSW, a corporate jet/ prop airfield for domestic charter and training for GA.