I know, a tree-structure allows a node to have several children and siblings, but just one parent. But for big boards, it would be really nice, to let subforums have more than one parent. In a nicely structured big board, you often find topics, that actually should be listed under different categories. But having two ore more separate subforums with identical name and topics is "suboptimal". Links don't really do the trick either, because they look and behave different... eg. they do not show, if they have new contents. It would be enough, if the additional parents would be secondary or virtual parents, because I think, other thinks in the node-hierarchy would be completely messed up, if there were really more than one parent.