I think it's a simple matter of choosing a few from the list as a way of expressing your unique interests.
Are we supposed to list 1-3 other majors from the list aside from our first-choice? What do they mean by "other subjects"? Math, biology, chemistry, that kind of subject?
I think you can choose 1-3 in addition to your first choice. And yes, you could choose any major. In addition to your first choice, you might be interested in social work, because a social work degree opens up so many different opportunities. Or, you might be interested in political science, because you are becoming fascinated with politics.
For your grad school plans, just tell them the current status of your plan. What specific areas of your chosen field interest you most? Whatever they are, describe some research involving them -- research you might do as a grad student. To whatever extent you can expound our plan, do so, because it shows your seriousness.
:-)