I think "What is Lucid Dreaming?" is the best question. I think the more general it is, the more you can right about it but... your teacher probably wants it more complicated right? That's kind of hard...you could do things like:

-What are the best techniques to achieve lucidity in dreams?
-Is it possible to control your dreams? (How can you control your dreams?)
-Is Inception a reality?

I don't know, just some idea off the top of my head. Hope you do great on the essay. Good luck