Don't get me wrong, I love Argentina (I was in Mendoza two weeks ago and the shopping was fantastic and the wine even better

), but it has less than a quarter of the population of Brazil and half the size. I think it would be much harder country to build a theme around. For Brazil, I think they could have a blast with a carnivale-themed pavilion. Food-wise, I'm sure you're going to get a meat restaurant with either country

.
But the US, Britain and Canada are already represented...personally, I would like to see more diversity in cultural backgrounds.
I do think Russia would make for a fantastic pavilion, though. I would also love an Indian pavilion since my favourite cuisine, hands-down, is Indian food.