I second your thoughts, and a plugin engine would be very very helpful.
I have been evaluating some frameworks, and so far,
http://jpf.sf.net is by far the best I found.
However, there is not - at least currently - any planned date to refactor JForum in order to support a plugin system. This is desired, but not done so far.
Rafael
[originally posted on jforum.net by Rafael Steil]