(I am in Australia.) To start with, don't make the opposite mistake: There is no guarantee you will end up working on the things you write in your research plan. A good plan establishes your general interest area and can help to find a supervisor, but after you have done that and get admitted, expect the research plan to be forgotten. One thing we use the plan for is to help us identify potential students who are really interested in something and have a passion for research. We know that someone with a passion for research can be guided into other research directions while maintaining the passion, so the precise details of the plan don't matter. The idea is to not give the impression that you are a clever person who is not especially interested in anything -- that's the student we don't want. Give the impression that you just can't wait to start discovering things.