If you want a purely useful language learn something like spanish, hindi, arabic, japanese, or mandarin. Probably russian and korean could go on that list as well. If you're in certain parts of the US spanish would top the list, and would probably be a breeze with your background in latin.
If you want something fun learn anything you darned well please, but german, italian, greek, russian, french, farsi, or any of the above languages.
As a linguistics major I'm going to attempt to learn the basics of as many as I can, and become fluent in two or three. Probably spanish, german, and if I can find a place offering it hindi. Mandarin scares the pants off me, or I'd take that.