Heide, I agree with you. Music could be one of the best. Subjects as music or PE have an incredible potential based on their intrinsic characteristics.
My choice would be a Science discipline (like Nat sci), to learn a second language! Subjects like Zoom, Botany, Chemistry, Geography etc require careful use, and hence learning, of language.
I think Maths is also interesting. Numbers are universal language, so a careful design of tasks can overcome the difficulties of the L2. Of course I am talking about concrete Math, in elementary education. Secondary is different.
I think the best way to consider what content area to cover for the purpose of language acquisition should relate to what the students themselves are interested in within your own ability to cover it. For example if a lot of your students plan on becoming engineers, then a math or science course/lesson would be appropriate, whereas is the students want to go into graphic design, then art would be an appropriate content area to cover. If there is uncertainty on what they want to do in the future, then just do the content area tht you think will catch the interest of the students the most, so that they think more about the content than the lanuage involed in it, but know that the language is how they can explore the topic further.