Do the classes you've taken count for any major (I mean, several classes counting towards one major)? Cuz if it were me, I'd just finish off that one major, if it's something I'm pretty interested in, and then use the spaces available in my schedule for a minor or to explore other interests.
If you plan on going to grad school, it's not necessarily advantageous to focus on just one field (unless you're sure that's what you want to study, and I guess it also depends on the field). Well, I'm not in grad school yet, but when I browse through programs, there are many (depending on the school/department) that don't even care what you majored in as long as you can demonstrate genuine interest in the subject. Like maybe you took 1-2 classes, read a ton on your own, and/or did some independent study/research on it.
Sometimes I also browse through CVs and profiles of professors at my school to see what they studied in college and grad school, especially in interdisciplinary fields, where I want to see if it's feasible to jump from subject 1 to subject 2 through post-secondary education. Like I saw how a professor did a Biology BS (with a philosophy minor), then got two masters in philosophy and biology, then did his PhD in history & philosophy of science, so I'd be like, hmm maybe I could try something like that. I also look at the years they graduate to see how old they were, about how long they took in the grad program, etc. But then a lot of these professors went to schools like MIT and Harvard for undergrad so I can't compare myself to them.
School rant: Stressed about senior project + grad school shit, but what's new? >.>