After that master class by Camille and Priscilla, I fear there's very little for me to add to this conversation! Damn, ladies!!!!
My paid stories have all been standalone short stories. I'm working on a novel, and hope it becomes a series. At the beginning, the protagonist has a life, a very few close friends, and a giant chip on her shoulder. By the end she's lost nearly everything but the chip, which has grown bigger. Here's hoping that's a good place to start a series, as low as you can go and still be alive...
As a reader, I have adored many a series--Holmes, of course, and even some of the follow-ons. Spenser and Hawk, Gar Anthony Haywood's Aaron Gunnar, Stella Hardesty by Sophie Littlefield, Lily Ivory in Juliet Blackwell's series, and of course the series written by my fellow LadyKillers! But I've equally enjoyed the one Sam Spade novel and many other standalones. As a reader, I'm omnivorous. So many mysteries, I've barely scratched the surface listing series I love to read!
Here's hoping my luck holds as a writer, and I get the privilege of applying the lessons Ms. Minichino and Ms. Royal have shared!