Just an FYI, the depiction of Aboriginal peoples in this game is innacurate. Their cultures were not cohesive enough to be considered 'countries' and the territory shown for most of them (Wurundjeri especially, their territory includes pretty much every other related Kulin group) are greatly exagerrated
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I would suggest that if you're able to you rather change the Aboriginal peoples to be historically accurate than remove them outright. Indigenous groups have been documented warring against each other even before European contact (the Bunurong and Kurnai), and there is evidence of trade which stretched across the continent. Both sound like features a playable country would need in this game.
Australian Aboriginals are underrepresented in media, but projects like your game could help others understand their history and culture.
I mean, I don't think they add much to the gameplay in this case. In EU4, the game the map is based on, they weren't fun to play or fight against, they were so inaccurate and a pain to deal with that I always disabled them. Anyway, the game is easily modifiable, so if in the future I or someone else wants to make a map of the tribes from North America or Australia, it would be easy to do. The map itself isn't the important part here, the idea is to provide a framework where any kind of scenario can be played.