Now I'm writing a sequel to a book I was happy with as it was, but it needs one or more sequels to improve its marketability. That's not to say there's no more story there. I like these characters, and I gave them enough backstory and left enough loose ends that they have plenty to do. But still, this time I'm the one whistling to get their attention instead of them yammering away in my head. It's a different experience. But I'm 14,000 words in and things are picking up, so I think it's going to be fine.
Actually, when I think about it, I had to write the other sequels, too, but for different reasons.
Oh, what a tangled web we weave when first we practice to market a book!