You should try to finish it yourself. You know where you want it to go, and asking this question with your qualifications is pure laziness on your part. It is YOUR story, and the compositions should be YOURS.
Serious writers don't ask what you are requesting. I am a serious writer, and if I have a half-finished story, I stick with it and I finish it. I have a story to tell, and I want to put it down MY way, not pick up another's thread, no matter how intriguing or complicated.
Try to finish what you have. If you can't. then the story will not work. If you want to write with a co-author, START with that person, agree on the theme, and don't go of on a tangent to finish it.
In case you don't think I know what I am talking about, I have written and had published several things - letters, articles, and other things. The best work of an author who is serious about being one is that what he or she composes.