I think of all the modern civs people suggest, Canada, Australia, Indonesia, Brazil, Isreal; Brazil is perhaps the most worthy to be included. It was an empire, after all...
Canada suffers from a crowded North America, Israel is too controversial, Australia and Indonesia are each comparably young, despite filling out a unfilled portion of the map.
Indonesia isn't young,if you consider them as a descender from Majahapit empire . They seem a good choice of Civilization and despite their importance,they never have been represented in Civ series before .
Tupi would be a good civ to add, I think if they don't add Brazil. If the Iroquois can make it in, why not the Tupi?
Tupi would be a really bad choice,because unlike the North American native tribes,they were just a bunch of hundreds of tribes,who didn't leave nomadic style of life and neither found cities . And unlike Iroquois,they fought each other even before the Portugueses stepped in Brazil .