Without a doubt, the most iconic mammals of Australia are the pouch-bearing marsupials. You can find less than 250 species in that one island-continent.
Marsupials have been around for 65 million years, and they are all that's left of a much larger clade called Metatheria, the entire bag of pouch-bearing mammals.
However, Australia is also home to a smaller, more ancient order of mammals--the monotremes, the only extant mammals to retain their reptilian ancestral trait of laying eggs.
In this alternate history scenario, the metatheres either never existed or have been extinct for tens of millions of years, leaving the monotremes as the next best thing. Once there, considering that monotremes were never ecologically popular (only 16 species so far identified in the group's 210-million-year existence), will the egg-layers radiate into a vast diversity of shapes, sizes and characteristics? Or will they just be incomprehensibly alien variations on the platypus and echidna themes?