A major airport to the South East would have to be out near Lang Lang down along the coast of Westernport. Any further towards the city, South or West of Kooweerup will bring the ranges into play for a North/South runway, which is essential in the Melbourne area. I can see heaps of opposition to anything near Westernport, esp the wetlands area, so most likely never going to happen. Then there's all the precious horse trainers in the area that can't even stand light aircraft, or anyone for that matter. Avalon fails, and will continue to do so until there is viable city pairs and it actually looks like a modern airport rather than some sheds, then there's all the issues with en-carriage and connections and rapid transit.