And another idea for a series...Have the same setting, and have the story continue, but with time gaps. The characters will be older, and play different roles. Perhaps even a new character will have the role of Protagonist. The best example for this one would be a movie, not a book...StarWars. From trilogy to trilogy, the Protagonist, Anakin, becomes the Antagonist, Darth Vader, while his son Luke becomes the protagonist.
And also in Star Wars the individual episodes jump. First Anakin is 10, then 18ish, then 24ish, Luke is first 18ish, then 22ish, and then a bit older again.