I would quite like to learn Mandarin. For one thing, I think that the wide net of dialects and varieties that "the Chinese language" encompasses are fascinating in and of themselves, with so much history, culture, and geography behind their distribution, and right now that information is less accessible to me than it is for, say, the Romance languages. For another thing, the process of learning it would be a good challenge, a means of keeping my mind sharp. Most importantly, though, it'd give me so many more people I could converse with! You're looking at a means of communicating with over 900 million more people (theoretically, ignoring things like geographical constraints, and that I can probably already converse with a sizeable proportion of native Mandarin speakers who know English as well)! That would be a wonderful thing in my book, so many more life stories and perspectives I could hear!
It couldn't hurt me to engage in more practice with Spanish. That's another good way to keep my mind sharp, and it would also open up the possibility of conversation with a massive number of folks. I could probably get by with what I know of it, and would be confident in getting by if I had a means of writing it rather than speaking it (I really need someone to, y'know, speak it with), but I'm far from fluent in it.
I admit that I also wouldn't mind learning Japanese, in large part because of videogames, though that's less of a priority than the other two.