I am a mixture. Sometimes I will get some ideas and jolt down the titles hoping I can elaborate later, other times I dive into my old forums and see if there is a discussion I could bring in my newer project. sometimes while in a forum I get ideas for a different one.

Most of the time, if I am going into a section thinking in creating a topic, before I open it I think what kind of things I'd like to discuss and contrast that to what is actually there. So for example, say I read this section "setup and management" before opening the section I try to think stuff related to that not even fully formed like say "damn I remember when I accidentally deleted my database" and that would bring me ideas for topics like discussing if people have deleted their database or done dumb mistakes, how often do they back up their DB, if maybe they want to share embarrassing first-timer mistakes, etc. Some of those threads will already exist or something similar.

I had a friend who would post threads about anything and I mean anything that came to her mind. I was amazed at how she would have daily topics many in different areas. So I try not to be too harsh on myself while thinking in topics and try to go by the no idea is too dumb to at least consider it. You can always polish the context later.

I just stick to the news, see if I can spin something discussion worthy.

It would be digging into knowledge already known about the niche.  I mean, take web development.   I can automatically think of topics like: "Do you prefer a VPS?", "What permalink structure do you prefer for your site?".