Based on your description, I think this will fit the bill perfectly for you, if you're still looking. It has a place to include an editorial brief for each story. It is fully collaborative, allowing you to invite multiple users/collaborators and give each one a different role with different privileges. It tracks who made what edits and allows for you to revert to any previous version. You can create campaigns and group stories by campaign. You can integrate with your WordPress or Medium site and publish directly to either or both on demand or on a schedule, as well as direct to social media accounts. You can review story metrics, as well, right from the dashboard. And, if you would like to find and hire a freelance writer to create the story, they have that feature in beta.