Australia has lost all its megafauna. We no longer have marsupial lions, four metre high flightless birds with half metre long beaks, or any of the other wonderful animals that used to roam. This has left us with a serious shortage of large predators capable of controlling kangaroo and sheep numbers, leading to catastrophic over-grazing and related problems.
It is possible that ancient DNA could be used to recreate some indigenous megafauna, but this is a long shot. So in the short term I propose that we introduce large mammalian predators to fix the problem. In New South Wales, we could simply release the contents of Western Plains Dubbo Zoo and find out by experiment which animals are equipped to survive in the Australian environment.