I agree with Fordman.
All stories, and story ideas / themes, on this site are subject to copywrite of the original author.
You can use the link in the story, to email the author.
Some authors are willing to let others write sequels. Other authors are not so happy. Many appreciate you sending a draft to them, so they can see what is to be published. Then the both of you, can mutually agree how to proceed.
I have written collaboratively with a few readers. I always credit such works with those who contact me, but I usually write it myself, then I have control what is said in my name. All authors have to make sure stuff sent to Gromet complies with the site rules.
At the end of the day, however, it is their decision and their permission that you need.
Hope that helps.
Daffy