Plugins can be nested. But currently in admin interface you can have only a list of plugins and not a tree (in similar manner like you can have pages nested). I propose that it would be possible to nest plugins (of course only those which support this) so that you would be able to define plugins which post-process other plugins.
Currently this can be done only so that top plugin provides some interface for adding plugins, and then the editor for those plugins has to be opened in the pop-up window. It would be much better if this would be provided by Django CMS itself. And interface would be then opened in the plugin space as for top plugins. (And those plugins already attached to existing plugins, like for text editor plugin, could also be then displayed in this tree.)
won't fix as 3.0 will handle this