Types of reader magnets

Currently I have one reader magnet, my short story "The Curse". I should create more, as I can use them to increase the number of signups for my mailing list. For example, I could write a shortstory or novella for the Iron Meridian book I am writing. There's several ways to do it:

1. Create a prologue of the main book (like "The Curse")
2. Use the first batch of chapters that provides 10k words
3. Write a separate story but in the same universe
4. Write something completely outside the Iron Meridian universe

Please let me know if I'm missing an option, I'm all ears!