I'm interested in working in a PhD research on a particular field. I've talked to a professor that despite his interest in supervising this work he would like me to come up with a few concrete research topics/objectives so that we could discuss about those and pick one.
I'm struggling to do so and considered asking here. I'm afraid the question is closed as off topic or too broad.
So: questions about possible research topics for a PhD are considered to be on topic here? Is there a particular way to ask such a question so that it becomes on topic? Or is it completely hopeless and such a question will no matter how asked be off topic?