Custom node type should be able to inherit from forum node

When you create a custom content type, you should be able to specify if the node should also inherit of the forum node type. That would allow any custom-defined node to appear as a forum thread if a forum taxonomy is defined for it.

Another possibility would be to implement an option (check box) in the publishing options of the submit form, along with the "publish on front page and snippet options". This option, if checked, would provide forum node behavior to any other kind of node and would display any kind of nodes (including story, book, page) as a forum thread if a forum taxonomy is defined for it.

Related forum topic I had initiated to try to solve that:
http://drupal.org/node/99611