Consider the required length and the level of development your teacher/professor want displayed in this assignment.
Having to guess at what you're doing and operating on the assumption that anyone asking a diablo3 forum for ideas is going to be in an undergraduate creative writing or grade-school English class, I think you'll probably be shooting for less than a dozen pages. That means your topic should be something you can summarize in less than a paragraph, preferably a sentence, and your development of said topic should focus on the general and the interesting rather than being thorough.
A random starting point probably won't be helpful. You should consider topics you have some familiarity with, or at least enough familiarity to write about them at some length. You can then draw up a visual aid of some kind (charting has always helped my ideas) and hopefully use that to bullet the kind of sub-topics you want to include in a way that isn't agonizing to read.
Of course, this is assuming you are choosing non-fiction. If you're making an attempt at fiction in the form of a short story or a poem, you are entirely at the mercy of your creative mind. Start doodling or making some kind of notes to break the stoppage on the river Id.