The outliner part of Fargo has been open-sourced. Although Fargo gives users the ability to post a headline and all of its sub-content to WordPress, it would be cool if you could outline right within WordPress. You know how there are already "Visual" and "HTML" tabs in the editor? A plugin could add an "Outline" tab, where Concord lives. Outline your blog post right there and hit Publish.
The downside is that your outline lives in some mysql database instead of an OPML file in Dropbox. But the upside is that the source of your content is in the same place where you publish it.
I don't have the PHP skills to create this, but I invite developers to run with the idea.