Here’s another tip for you serial writers: Don’t forget your minor characters! You never know who might be your readers’ favorite. Believe it or not, they often get attached to supporting characters as much or more than the protagonist. Thus, if a minor character goes away, they’ll be disappointed if they disappear from subsequent stories.
Another thought is that popular minor characters make good fodder for spinoff stories, whether as a short story or another novel. When my fans told me how much they loved Thyron, the telepathic walking plant in my Star Trails Tetralogy, I wrote up his background in a short story that I give away. In addition, I eventually wrote an entire novel with him as the central character, which in the beginning I’d never thought I’d do. However, his popularity warranted the effort.