Custom stories should have good atmosphere, good puzzles, very good mapping and detailing, custom stuff(lots of people like to see new stuff in a custom story) such as custom music, custom entites etc.... Don't make to much jumpscares, you don't want a custom story just full of jumpscares and that's it!
I don't like custom stories where all you have to do is find keys and open doors and find more keys, and last but not least is a good story! A custom story should have a well story, and don't forget to be creative! that is all the tips I can give you
Goodluck with your mod/Custom Story!