There were proposals to make a one-gauge line to the Barrier (Broken Hill), however NSW wouldn't allow BG into NSW. Stillborn.
The were proposals for a BG Land Grant railway across the Nullabour Plain between SA and WA, but this did not get off the ground.
On the other hand, the stupidity of having a Break-Of-Gauge "only" 40 miles from Adelaide would have been solved by extension of the NG from Hamley Bridge to Adelaide and Port Adelaide. Never got off the ground.
There were also proposals for line between South Australia and Queensland, of NG, and probably via Birdsvale.
In the 1920s, there were Federal proposals for a SG line from Port Augusta to Hay. Vetoed by SA and VIC, because it bypassed Adelaide and Melbourne.