I have a lineage that traces back to both Irish and German. Therefore, I would like to know German.
Not only do I want to go back to te "old country" but also because if (or "when" if you're anti-American, I'm not for the record, I love my country) the American economy goes the rest of the way down the toilet, I could set up shop in Germany, which is the largest economy in the European Union, and only smaller in GNP than Japan and the US (based on purchasing power parity).
Irish Gaelic would not be so important to me, mainly because English is so widespread in Ireland, so I wouldn't have to worry about that so much when I go there. But it would be cool to speak a language that is so rare comapred to your well-emphasized Spanish and French in American schools.
Spanish, despite my last comment, would also be very helpful, since there are a lot of Hispanics in the US. Plus, I already had four years of it in school, so it wouldn't be to tought to learn more of it again.
Hebrew would also be very interesting to learn. I have Jewish friends and have wanted to go to Israel for a very long time.
Japanese or Madrian Chinese would be helpful, since Japan is second only to the US in terms of the economic scope. Mandrian Chinese is more common than Cantonese, and is not only spoken by the (People's Republic of) China, but is also spoken by Taiwan (the Republic of China). Both nations have powerful economies as well.
French is just so damn cool. Even if it's not an official language for many countries, a lot of people know it as a second language.
Latin would be good because I am Catholic and before the 1962 Latin Rite, the liturgy was read and spoken entirely in Latin. My grandfather was an altar boy in his youth, and he had to learn all the Latin responses. Plus, since it's a dead language, its not going to change and there is only one dialect.