Right now, I'm using the "Someday" slot in Things to represent concepts, ideas, and other things that don't have an immediate "plan" attached. It works well enough for storage and search, but that "assembling together" process has brought up another idea for something that would work better: basically, a program that shows you all your ideas as little fridge-magnets, allows you to move them around and draw relationships between them, and group them together under "named entities" (e.g. for a novel, character traits could be moved around and grouped to form characters.)