Like Dolby said, start off with some small Addons. I wrote an Addon specifically for that purpose, the
Generic Addon (I know, I know, such a generic name!), that contains just enough to get an Addon to load and create a /slash command.
Then, you can move up to more complex Addons that create frames, which are used to display stuff on screen (not just in the chat log).
The wiki here is a good place to start. Your Addon idea is interesting, but the API for it isn't developed yet. However, that doesn't mean you can't work on it......after all, you need a framework for the user to interface with, and you can build that now =)