In my opinion Africa will be hard to do accurately given the variety and number of the various local tribes. Asia will be even harder, probably impossible, given the various Asian cultures and how different their architecture and units would look.
On the plus side you can add in the whole of South America without being too incorrect. Just fill it with Spain and Portugal and it will be more or less accurate to history. And Australia is practically deserted at the time. The aboriginal tribes did very little fighting, so representing Australia as more or less deserted would not be too inaccurate.
There are quite a few places that were occupied by a faction already in game that can be added in without problems. A lot of island chains (especially tropical ones) come to mind.
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