I like themes.
By theme we're talking about a related set of issues within the topic. A theme is something the community discusses in depth for a period of time.
If the community discussions have become stale or repetitive, use your subtle influence to guide the discussions in to a new theme.
You need to initiate topics about important issues, slowly evolve the discussions to related issues, and create associated content. It should all be linked.
A community can go through 2 - 3 themes a year.
Initiating a new discussion is fine for a quick blip of activities, but having a regular programme of themes for the community to discuss sustains a high level of activity.
Don't jump around from one topic to the next, move through a related set of issues. Move through unique themes.