When do you write the sequel?

Is it time to branch out into an all-new novel or march on with a sequel? I have several project ideas waiting in the wings, but I have also been making notes for sequels to two existing, but unpublished novels. Somehow, the idea of a “part II” seems premature when I don’t even know if anyone will like the first book. But I will likely need to have characters or situations that set up a sequel in the first installment of a series, which leaves me wondering whether the initial novels are truly finished.

This seems particularly important for my time travel series. While the world I have built does not allow looping back to the traveler’s starting point, it is still more complicated than a normal linear story line. I have already gone back into the first book and tweaked some facts based on my notes for the sequel. How much more will I need to change once I start the actual writing? The risk of omitting something critical to the later story makes me feel that having the complete series in front of me would be wise before I try to publish the first book.

I know popular authors write sequels long after publishing the first novel. There’s nothing to stop me from doing the same. I am creative enough to knit two stories together after the fact. But how much cleaner could the seams be if I sewed them up tight right from the start? Even straight-line stories depend on the original framework and setting of the story. Would it be best to stay immersed in the world I’ve created and spin another yarn? And how about a prequel?

Am I overthinking all this?

Writing the entire series before publishing any of it may be less important for novels where there are no backward time jumps. I’ve got more than one series started, so it might be useful to let a book sit for a while and work on something else before I write the next installment. It could also be better to stagger the writing and see if I get more perspective by waiting to move on with a storyline. But how many years does that turn into? I can write fast (not Stephen King fast, but fast enough), but as the unpublished manuscripts pile up, along with all the other work I need to do to get the novels to market (not to mention trying to build a customer base), the sequels could turn into a years-long quest. 

But these incomplete stories tug on me. They nag so loud that they’re hard to ignore.

Damned whiney sequels.