Say I create a copyrighted written work, is there a license I can apply to that same work that will allow a third party to create and copyright a derivative work? Is there an existing license or copyright type that would allow the creator of a derivative work the ability to copyright their work, but only if it is a sequel and not an edited version of the original written work?
You have two ways to go here.
Add to book notes or social media or whatever you wish that you are open to people creating sequels and will grant permission to do so on a case by case basis. Then you have full control over who can write a sequel (and can weed out things that people call sequels that aren't).
Use an existing license like Creative Commons. Here, you have less work up front, though you still need to check to make sure people are using the license responsibly. And you have less control over what gets allowed.
A couple notes:
- Your work is already copyrighted from the moment you wrote it (at least in the US and many other countries). You have to explicitly transfer copyright or have the work enter the public domain to avoid being the copyright holder.
- Instead of saying "sequel" you may want to say "story set in the same universe."