In wiki systems, I almost always use outlining - using lists and sublists which most wikis support. The key difference is that you normally have independent topic for each thread. And these are not necessarily connected (i.e. available from one UI). One approach, for e.g. in twiki, is to use 'Idea' or some such word in topic name, and use search to collect and display all such topics in single page. But it is not quite same as what we want ideally: Have all threads simultaneously available.
Second problem is that wiki systems require you to be online by default. But ideas come primarily while you are offline, and it is easy to take out your pocket PC and put a line or two as notes. You require good synchronization capabilities to make this happen, and with a tool which makes its outlining data available in open format.