I haven't been playing long, but I've found that when building a sizable fortress, you soon end up with quite a lot of levers. You also end up with a number of future plans, and little details you want to make a note of, for example, where to leave a section of rock for a future channel.
What I'm suggesting is some kind of method of putting down small signs or notices, which we can refer to at a later time. It could be an object you can build in a workshop, a small wooden, stone or metal sign you can tell your dwarfs to place, or perhaps just a menu option that allows you to place and view notes that don't affect your dwarfs, but act as a reminder to you.
I could see this being especially useful in games which are played by a number of people. It would allow the players to transfer little notes about their future expansion plans or what various things do. It might also be useful if you come back to an old save of a large fortress. Blindly pulling the levers of your state of the art magma defense system to remind you how it worked will probably result in problems for the people on the lower workshop levels...
But enough dithering. I'll let you think of what it could be used for, or how it should be implemented, and provide some feedback on what you think of the idea.