When I write, I make an entire story. I normally don’t think of my stories in a franchise format. I know that there are many books with many stories. That is something I have never attempted yet.
There are many sequels to movies. Sometimes during the summer season every title that wants a box-office draw has a 2 at the end of it. Sometimes the movie industry banks off the success of the initial movie only to bring out another inferior product. There were many movies that I became disappointed with when 2 came out, and because of the stellar original, the second one just took my money. I would never do that to my readers.
If I’m not thinking about a series, I won’t force a second book. Being independent can be liberating if you’re not a Stephen King. Stephen knows all too well. Read Misery(tm).
If I get an idea, I watch it in my head like my own personal movie. If it’s a movie I would want to see, I write the tale. I’m not saying that I wouldn’t do a series. I’m just saying that if the story is finished, no matter how successful it becomes, I won’t write another where my characters become impregnated with fluff. How many of your amazing movies turned into dreck for it’s sequel? My characters have been painstakingly created, and they don’t deserve dreck.