Looking at the API, it seems that we can access a story's prequel, but there's no way in either the Story Resource or the Story Endpoint to get/search for sequels?

For my use-case, I was considering either enhancing the existing Calibre plugin FanFicFare, or introducing a workalike plugin for Calibre, using the JSON API instead of web-scraping.

Page through every story on the site, inspecting their prequel value!

6114797 One could even implement it as a service—a database of sequels, with a public API.

Might I suggest adding it to the /stories endpoint, as a filter?

filter[prequel]=#id

Another option would be to have another relationship sequels that would be a list of stories that are listing the present one as a sequel.