This question asks for guidelines on what to write about. In the realm of fiction we have already declared questions asking generically for plot ideas off-topic. In the linked meta question HedgeMage shows how questions asking more specific, focused questions can be on-topic.
Questions asking for what to write about in an essay seem to be a non-fiction version of asking for a plot idea. Are they also off-topic if they are not the tightly focused type of question that HedgeMage proposed?
I would propose that questions about what to write about, in any genre, are off-topic. A question that tries to hone an author's existing idea, on the other hand, should be on-topic. What do you think?