Firstly Melbourne airport should extend RWY 27 and yes it can be done. That would be a quick fix to allow more heavies to use it. Otherwise any extra runway would be good.
The original plans for YMML allowed for extra runways. Runway 34 has extra fillets at the southern end presumably because someone in the 1960's had the foresight to put them the early to save effort later.
Brisbane also had 2 parallel runways on its master plan. It's only taken 30 years to build it.