If you’re writing your first book, you might resent my even suggesting this. You’ll be happy if you can make it through the stress of creating one coherent, meaningful, interesting, readable novel. And I’m suggesting you write another one—maybe several other ones? What am I thinking?
Well, I’m thinking about your future and your pocketbook. Many authors today are settling into more comfortable lifestyles because of the proceeds from their mystery, crime, adventure, children’s, etc. book series. Think about it, if readers like your first book—if they enjoyed the story, found the characters engaging, like your writing style and the theme of the book—they will undoubtedly buy your next book and the one after that and so on.
If you have a good idea, why not capitalize on it? If you’re promoting one book on a certain theme or in a certain genre, why not promote several? You’re in the zone, anyway. You know how to promote books of this type.
Should you write a series? If you have strong characters, a good story idea, and you can tell that story well, I urge you to consider planning a series. Publishers like them and readers like them. And these are definitely the people you want to impress.