I don't see the United States becoming a full-on social democracy like Scandinavia but a transition to a social market economy with many social democratic elements like in Germany is quite possible as Millennials rise to political prominence.
I also see this possibly happening. The Millenial generation may be libertarian on social/civil rights issues (gay marriage, weed, wiretapping) but economically they are much more liberal. I could see our health care system becoming similar to Switzerland (basically a more liberal Obamacare), and higher education being paid for by the government in the the next 30 years.