I refer you to this post: http://www2.adult-fanfiction.org/forum/index.php/topic/52596-want-to-adopt-a-story/ and suggest you go through the forum, to see if you can find some stories that fit your preference.
And I'd like to suggest that the best way to get better at writing is to just write your own stuff. Writing is a skill, and it only gets better with practice, and the best practice is to make up your own stuff, not work off other people's stuff.