Hi,
First of all make sure you want to an EPQ and if it will really help you or not. EPQ is extremely difficult and time consuming and in my case (for medicine) it doesn't help much. So if you're sure, then you need to make sure your question has some sort of debate to it. Something where you can argue two or more sides so that the essay is evaluative. You need to make sure it's not descriptive. Remember that your essay doesn't count for that many marks compared to your production log. So you should really focus on the planning part of your EPQ. You really have to show your thought process throughout the project and be reflective in order to gain the most marks. And for more specific advice, make sure to extensively research your topic once you've made a plan. I don't know if I have really answered your question, so good luck and make sure to ask if you have more specific questions!