The research communities are too broad and too deep. First you have to choose a broader field such as mathematics or physics or chemistry or biology. You could

The research communities are too broad and too deep. First you have to choose a broader field such as mathematics or physics or chemistry or biology. You could also think about inter-disciplinary fields which are considered "cool" in the last decade. It has to be a mix of something that interests you and something that you are capable to learn. This is an arbitrary and naive question. I can tell you to work on Millennium Prize Problems but is it really a relevant or helpful answer? I think not.