Why isn't it based in Great Britain? Add the Ferry and tunnel to France.
All the different cars, buses, violence, old architecture, castles, art heists, snow, etc.
Tokyo has all that as it is.
Australia would be pretty much what GTA V looks like now.
I remember playing the 2d GTA, plenty of fun back then. Don't know why they abandoned England.
Australia could be interesting. Several times there are mentions of a war between the USA and Oz in the GTA universe. Exploring what led to that, and the fallout from could be very interesting, if R* is interested in building a fictional universe and a history for it.
I picture they could represent Australia in one of two ways:
Either by showing it as a lawless society of godless heathens and criminals, playing off its history as a penal colony. A land where the police and the government have no control over the populace. Gunfights and street racing breaks out in the streets, civilians constantly having to keep their heads down to keep themselves alive. World record highs of crime all across the board. Perhaps that's what instigated the Australia-America war, Australia's lack of society led them to try to invade New Zealand or Hawai‘i (one being a US ally and the other being a US State as in our reality), and America retaliated. Due to Oz having no formal military or training, the American army stomped their invasion out almost immediately. The main character fighting other criminals that effectively run the country for his/her own benefit and also to try to protect those near and dear to them.
Another idea is the exact opposite: portray it as a parody of what Australia is now. A hyper-sensitive nanny state where the government is all knowing and all powerful. It watches everything you do, and polices everything the people do. A totalitarian dictatorship where any individuality is snuffed out. Crime is mostly white-collar, and done from the shadows. Anyone with power is corrupt. And perhaps the reason for the war was particularly greedy Australian government officials tried to steal from say the American federal reserve. Obviously a good cause for a war. The main character mostly fighting and stealing from the government.
Either way, a good mix of city-scape and outback would be good. The main city being a mix of Canberra and Sydney. Rainforest based upon those in Queensland. Steep snowy mountains for the Australian Alps. Large swathes of desert scrub and bush for the Outback where you can do anything you want without incurring a wanted level. Though you have to watch out for the wild animals. Crocodiles lurking in rivers and lakes, Kangaroos that suicide into your car totalling it and kicking you to death if you're on foot. Drop bears that fall from gum trees and maul you. Poisonous snakes. Capability of dying from dehydration and heat exhaustion. Beautiful ocean for diving with fantastic corals and reefs, and hundreds of sharks waiting to tear you to shreds.
Wouldn't even be that hard to come up with names for the states and territories. New South Scotland, Elizabeth, Kingsland, North Australia, Southern Territory, West Territory, Dervonia/Deimenia.