To make a good expository essay topic, you need to have some sort of question you are answering. Additionally, I'm not sure you are specific enough in using the term "Eastern Religions." Which religions are you referring to? Many religious groups would not be comfortable being grouped together. I'd encourage you to pick a particular religion to talk about. I'm assuming you are referring to Islam, Buddhism or Hinduism. Perhaps there is another one you are thinking about. Here are some examples you can use on your topic, but I'd suggest you substitute a particular religion for "Eastern Religions":
1. How are Eastern Religions changing American culture?
2. Are Eastern Religions damaging the culture of America?
3. Will Eastern Religions be assimilated into the culture of America?
4. How are Eastern Religions being assimilated into American culture?