I don't do more than 3 or 4 books in a series, ever. I create so many unique worlds, characters n critters, I have to keep a chart as I write. I don't do formulas, so don't expect to see book #19 from me in this lifetime.
I'm different, and I love it. It's deeply satisfying.
As for creating interesting characters, I'm so glad readers love them. The thing is, other authors do that and then end up plugging them into tired old plots. Readers deserve better. I'd rather stay fresh, and besides, I have so many different worlds swirling in my head, I might explode if I don't get them written.
Where do I get my ideas? I fight them off with boat oars so I can finish the books already in the dock.