I know there is bbpress, but I hope it will be supported here. Below are some of my idea with bbpress.
Allow the forum(s)/topic(s) created in the main site to be able to place in any sub-site(s) or any webpages (wiki, post, page and etc.) within the main site. That is, users need not visit the forum(s)/topic(s) at the main site for discussion. They can discuss them at any places where the forum(s)/topic(s) have pulled in from the main site. Thanks.