I think people are forgetting that Luna Park Melbourne used to be home to a 2nd coaster in the form of the Big Dipper. This was added to the park in the 1920's and the Sydney example was a clone of this coaster. There is certainly more than enough room for a 2nd coaster within LPM- especially within the footprint left by the former Metropolis coaster, which in itself is in the footprint of the former Big Dipper. At one stage , the Big Dipper was slated to be replaced with the Titan Vekoma Bomerang coaster from Expo Park before the deal fell through and it went to Wonderland instead.
There are plenty of coasters that would be a perfect fit for the park within that footprint including the aforementioned Zamperla Motocoaster, Maurer SC 2000 Spinner, a custom Gerstlauer Eurofighter would be ideal , a Premier Rides Skyrocket II, a Custom Mack Big Dipper coaster, S & S El Loco , Maurer Skywheel, Skyline attractions Skywarp, Zamperla Tunderbolt , S&S 4D Freespin could all concevievably fit into the park with some vision and planning.
There is actually more usuable and flexible space to fit in another coaster at LPM than there is at LPS. Another coaster that has moderate thrills would be just the ticket for the park to add to its list of attractions. Some of its current ride roster such as Enterprise and Twin Dragon are getting older so some thoughts as to replacements would definitely be on the horizon. Obtaining another coaster at this time seems more than sensible and a coaster such as a compact Eurofighter would certainly catch the public's imagination. The Eurofighter model would be for mine, the best option , but a Sky Rocket II would also be an excellent addition.
Have a look at the coasters below that could all fit into the space required at LPM - any of these would be great addition to LPM's lineup and should be well within their reach both financially and physically to obtain and fit into the park.