Aztecs,Iroquois and Incas have been in Civilization series since Civ3 at least . And like I said before,it's a moot point,because Aztecs isn't competing with Brazil for the 9th slot . And I doubt of your position of finding them boring,because you'd never know which UA and UU/UB/UI they would have if they won't be added .
As I say, gamewise they could well be interesting - I wouldn't have picked Austria as a choice of civ, though as a Great Power when the concept was developed and for some time afterwards, they have a reasonable case. but gamewise I'm intrigued by them (and love the leader screen background).
But any civ can be given any game mechanic - look at the Songhai, whose mechanic doesn't obviously reflect the real-world civilisation, but is interesting gamewise and has added a moderately obscure civ to the Civ pantheon. They could easily have been named after goodness knows how many other civs, from various parts of the world - and in their own region the Nigerian Oyo would probably be a more suitable representation for the same effect.
The reason people are discussing "what should be the next civ" is entirely because the names and backgrounds to particular civs interest them, not the game mechanics. You yourself mentioned in Brazil's favour elements of its native heritage, cultural diversity and slight differences from European cultures. None of that would be reflected mechanically if Brazil were in the game, but that doesn't prevent people wanting a Brazilian civ.
For me, Brazil is simply not a particularly interesting culture, any more than Austria is, and I wouldn't find Brazil as a society as interesting as, say, the Nabateans or the Champa - or indeed the Sumerians - even if they were given the most original ability in the game and became my favourite civ to play. My preference for civs to add to the game would be ones with a longer history or past civilisations with a particular cultural legacy, those which aren't especially well-known and, as a personal bias, particularly ones from Asia.
Hmm, for a Brazilian UU how about:
Sertanista (replaces Scout): Improved chance of getting 'population to settle in your city' results from Ancient Ruins. When encountering barbarians during the Brazilian turn, has a chance to make peaceful contact (i.e. barbarians will not attack the unit if peaceful contact is made).
