Auto-Archive old, stale or uninteresting ideas
For ideas that are "old" or not popular with the community, automatically archive them after a specific period of time.
For example if it were know that ideas that receive no votes or comments in a 90-day time window that these ideas would be automatically archived, two things would happen.
First, the clutter automatically cleared out of the way so that one could focus on fresh ideas. Second, there would be a built in promotion component. Whomever was the original poster would be incented to continually recruit new participants in that specific idea or even recruit new community members.
Naturally if you did this, you would need put a timer on ideas that are getting within the 2-3 week range so that the author could get back out there and start promoting. And any given post or comment would reset the 90-day timer.