That's a tough one. Obviously you want to do something that you are really knowledgeable about. My brother was writing an english paper the other day and he wanted me to pick a title, and trust me it is harder than it sounds. As for helping you pick a topic, I think you already did. You said you were very interested in network and wireless security. So just do something with that only make it more specific. It is very hard for us to give you an answer to this question, ultimately your going to have to decide the topic and it won't matter what we think you should do, only what you want to do will matter. So just trust yourself and pick your topic.