I know how you feel. First, let me say when I was an undergraduate I hated proof base classes. Thinking about abstract algebra still makes me sick. It's part of the reason why I emphasized in statistics. (I thought stat didn't have any real abstract level math.) Sadly, in graduate school, most of my statistics courses that dealt with theory were heavy in mathematics. I had to take functional analysis just to finish my thesis =[.I don't like the thought of having to specify a major. I enjoy having an understanding of everything and don't like limiting myself in that regard. I have no idea what I want to do with my life either so this makes choosing even more difficult.
Anyway, the reason why I quoted this part is because it makes it hard for us to help you. I'm clearly bias toward a person taking probability theory, because, imo, statistics is a field where you don't have to be a subject matter expert in a narrow field to be helpful to a myriad of disciplines. This has allowed me to jump from finance to bio-stats to quality control for a factory and then back to bio. However, just because I think it's a good idea, doesn't mean you'll enjoy it. So at this point I think any math elective that seems interesting to you is a good one, if for no other reason than to teach you want kind of math you like and what kind you dislike!