Riis is a great idea.  There aren't too many people who are important in our history who came to the US because they were love sick over having been rejected for marriage.  By a girl he first met when he was 15 and she was 12, no less...  Interesting way to remind us how "great men" are just as human as everyone else.

As you say, Danish immigration would be an interesting topic to study, they are an often-overlooked immigrant group. I know that a large contingent of early Mormon settlers were Danish immigrants. Also, one of the most important Americans to comment on the living conditions of "new" immigrants in inner cities was a Dane, Jacob Riis. So I'd say that would be an interesting topic.

I like #2. I've heard a little bit about Japan's motivation for expanding in the Pacific for oil and/or economic reasons. They may have considered us a threat to their expansion, so taking out Pearl Harbor made strategic sensen for them. It would be interesting to get it from their point of view.

This is quite a bit different than some of the suggestions you mentioned, but I'll offer it anyway.  I have always been interested in studying the interwar period from the Asian perspective.  Almost everything I have studied or been taught about the period focuses on life between the wars in Europe or the United States.