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A world like this? I like how the Pacific keeps its shape weirdly. The South Pole is at 20S 133E, which in real life is in the middle of nowhere in the Northern Territory, 500 km north of Alice Springs, 800km south of Darwin, and in between them longitude-wise.
Also makes you wonder about climate. I'm not an oceanographer, but can the deep trenches/gaps in the continental shelf around the Wallace line sustain a strong circumpolar current as exists around IRL Antarctica? That current is a big part of why Antarctica is so cold and plays a big part in Earth's climate.
It seems perfectly possible based on this, assuming sea levels similar to real life's. Still, it's no Drake Passage. There doesn't look like there will be much glaciation in the northern hemisphere either (in my opinion just based on the lack of land near the north pole), and Australia has a smaller land area than Antarctica, so maybe sea levels might be a bit higher anyway guaranteeing a strait for a strong circumpolar current? Because I know the Indonesian continental shelf was exposed during our ice ages.
All in all it looks (in my opinion) like the world's climate characterisation is either going to be somewhere between "a bit more maritime and balmy" and "Pliocene".
Marsupials should be fine, since we know from genetic and biogeographical evidence that Australian marsupials crossed to Australia from South America via Antarctica in the early Cenozoic. Antarctica will almost certainly have Australian-like Marsupials living there if it doesn't freeze over. Honestly, looking at its geography here, I think it could have a bunch of unique species living there in the present day. The Drake Passage isolates it from the rest of the world even after South America gets a faunal interchange, it's like 1000 kilometres. It could even be another Australia, with the only native placentals being rodents, bats and marine mammals, just bigger, more mountainous and shifted a bit north.