You should write on an unsettled question that interests you enough to keep your interest for a long time. If you don’t have a clue what this is, then I recommend you do some literature review. Research is not done in a vacuum. Once you know what conversations people are having, then you can see where your perspective can make a valuable contribution.
The modern resurgence of Stoicism in the zeitgeist is exactly this: self-help without actually engagement with philosophy.