I posit that anyone who has written an addon that they think would be eligible for extensions should have an official Forum thread dedicated to the discussion of its API. I know there are many out there which can be extended upon (via LDB, FuBar (hopefully someday LDB), etc), and which currently have no documented way of doing so and are thereby "find it and make your mark".
Main contenders, AFAIK: oUF. I know Haste has decided to move the official thread off of the Ace forums, but I do know I saw an informitive and lengthy thread on the howto and how-not-to for it.
My thinking is that other addons (whether or not their author realizes it) may benefit the community as a whole by having a discussion of their (existing or yet possible) API and possibly having it incorporated into a Wiki.
Exactly. If an addon is written with an API for other developers to hook into and use, the addon's author should write at least some basic documentation of that API and at least link to it in the addon's main description. I don't think there needs to be a separate thread for API discussions; those can take place either in comments on the API documentation page, or in the main forum thread for the addon.
Also, this thread is not about a specific addon you are releasing, and should be in the General Chat forum. Moving.