As Capnvonbaron said, civs go through various cycles. So you can make America expansionist by looking at that early settlement period. But then you can make any European nation expansionist, because they've all had their time of exploration and colonization, and you can see early America as an extension of that, and not something specifically American.
Where for me America truly stands out - apart from commerce - is communication. Because there have been so many people making up the American population - Italians, Poles, Irish and what not - they had to really work on communication to understand each other, and as a result developed a quality that you won't find anywhere else. It's both communication technology and culture. A lot of American culture is hugely accessible and influential. And then I'm certainly talking about movie pictures and things like that, about Steven Spielberg making more accessible movies than Rainer Werner Fassbinder, to mention an example. It's just not so easy to capture this in one of the traditional traits, and maybe Americans aren't too chuffed about this quality of theirs either, and rather see themselves as religious or agricultural. Perhaps a bit more time is needed for Americans to develop an identity that corresponds better with their true qualities.
Pyrrhos Scandinavians I can hardly not give the seafaring trait, but apart from that you can throw almost anything at them. Probably the Viking legacy is what most people remember Scandinavia by, and that's easy material for a game, so perhaps that's why Firaxis have gone for expansionist and militaristic in PTW, and swapped expansionist for seafaring in Conquests. A game like this can only use a charicature.