I have settled on some things. I haven't talked much about worldbuilding. Really, I think research and worldbuidling for certain projects can need much more time than one month. For this project, though, I'm world-building as I go.
Since October Knight is going to be a series, I have to keep in mind which elements of this book are going to be part of the Series Template.
Template is a term from television screenwriting. Every episode in a TV series has to conform to the rules of that series. These rules go beyond genre. For instance, both Dexter and Medium are detective stories. Ghosts and prophetic dreams not only can appear but must appear in each episode of Medium; in the Medium universe, ghosts literally exist (though only mediums can see them) and dreams convey facts (though often misunderstood) about the past or the future. Dexter sometimes sees the ghost of his dad, but this doesn't happen all the time, and when it does, it is not meant to be understood as a literal ghost, but as a projection of Dexter's psyche.
Those are worldbuilding differences, since Medium has an element of fantasy, whereas Dexter is more "realistic." (Not really perhaps, since we still have to accept that a psychopathic mass murderer can be hero, but there is no "magic.") Each show has its carefully drawn parameters. Medium has ghosts but not vampires.
In addition to the worldbuidling differences, there are other equally important parameters. Both Dexter and Allison DuBois (heroine of Medium) have families, but the show rules treat their families differently. On Medium, the unspoken rule is that nothing bad ever happens to Allison's family. If she has a prophetic dream that the serial killer of the week kills her three daughters, it usually turns out that the dream really refers to another family. Dexter's family has no such guarantee of immunity.
I'm not sure how many Holiday Knight novels I want to write. In addition to the Halloween story, I'm setting up characters for a Christmas and a Valentine's Day story. In theory, I could do as many as twelve. (What is it with me and series of twelve? Sheesh.) But I don't want to get locked into that, so the first rule I have to set for myself is to make sure each book can stand alone, yet leave room for future novels in the same world.
Here are some other important things to decide for a Series Template:
Characters: Does the series have the same protagonist and supporting cast? The Unfinished Song has the same heroine and hero (Dindi and Kavio) and recurring villains and supporting cast through all twelve books, although Dindi is the anchor character. However, my Holiday Knight series must have a different protagonist for each novel so that hero's story and romantic subplot can be complete. I will use a technique common in Romance series, where supporting cast in this book can become the heroes of heroines of future books.
PoV: PoV should be consistent across a series. That doesn't mean each book has to be about the same characters, but if one book is multiple third person past tense, they all should be. I think I'll be going with first person past tense for this book, which means that each book will be first tense, but from the PoV of a different protagonist. That means that the Voice should change with each book--but not enough that it is out of keeping with the overall Tone of the series.
Tone: This is not the same as Voice, although its related. Tone is the name I give for those rules which determine if a character's kids get killed by a serial killer or are, by unspoken rule, off limits. Humorous or series? Drama or melodrama? Cussing, rutting and bloodletting fit for HBO or Disney Channel clean? This is something that might not come out in the early Outlines, so the Seed Scenes are important.
I kept my Outline clean and funny, but my scenes are turning out to have serious situations, and risk being more gritty than I want. One possible version (this could change) has my hero homeless at the beginning of the novel. Although he's not a user himself, he lives in what is basically a crack house, because those are the only friends who would take them in. I realized that if include nineteen year old junkies and forteen year old crack whores in my story, it's probably never going to be made into a Hallmark family holiday movie. And I was really looking forward to that.
I may compromise with a tone of Dark Humor. There's gore, but it's funny. So much depends on what I can actually pull off.
Time: You couldn't have an episode of 24 that covered three months (excluding flashbacks). Most series Templates aren't so strict, but still, I find that one thing which can make a trilogy or series of novels feel "off" or unbalanced is if the writer loses control of the passage of time. I remember one otherwise good sf series where most of the story action took place over a few weeks in each book, but suddenly in the third book, the writer tossed in a two year journey in Chapter Three, because the two characters had to travel between planets and they had no cryonic suspension or FTL. If they had been asleep, the two years would have worked. It was the fact they spent that time awake but Offstage that snapped suspension of disbelief. In terms of character growth, the writer treated this two years as if it were insignificant. The story continued after the journey as if the characters hadn't changed from the last time we saw them. Hm, shouldn't they have changed a lot more in two years, especially under such isolation? It blew the whole story for me.
In The Unfinished Song, I cover about a year per trilogy, with most of the time passing in the first two books and then events transpiring quickly in the third book of the trilogy. For instance, in Root (book 4 over all in the series, but the start of a new sub-trilogy), about 9 months pass, in Wing (book 5), three months pass, and in Blood (book 6), everything takes place within three days. There will be a big climax, and then in Book 7, things will slow down again and the better part of another year will pass. Book 9, third in the third sub-trilogy, will be the Vaedi Vooma (the dance to choose the Vaedi) and everything will happen quickly.
Plot and Subplots: Lois McMaster Bujold alternates her Vorkosigan series between mystery and romance subplots, under the umbrella of rollicking space opera adventure. There's an advantage to that when you're following a single character, in her case, Miles Vorkosigan. In early books, Miles has romantic flings, but they dissolve, and it's okay, because he achieves the goal of the main plot. Finally, some love comes Miles way in what is, hands down, the best of Science Fiction and Romance ever written, A Civil Campaign. But after the HEA (Happily Ever After), how could she write another book about Miles? A lesser author would do some lame thing like have his wife killed by an assassin (or turn out to be an assassin) and send Miles on a rampage for revenge. Instead, Bujold has the newlyweds act as detectives in an awesome space mystery. She simply switched sub-genre, and it all fit.
Mixing up the subplots can keep a long series fresh. In general, though, it's a good idea to define the extent of your subplot territory in terms of the series Template.
What that means for me is that I intend to keep my main hierarchy of plot-subplot-subplot. Each novel, like the first, will have a story of a teen earning the power of one of the Knights of the Year. Each novel will be associated with a holiday (although if I do weird months like June, I'm not sure how I'll handle that). Each novel will involve a romance that has a HEA. Possibly each novel will also have a murder mystery... I'm not as certain on this point. Definitely each novel will be funny, or at least as funny as I can make it, and funny in the same tone as the first.
Theme: I called this category Theme, but Theme is a bit more amorphous than what I have in mind. In a series, particularly if it's episodic, one needs to know what force is going to generate the conflict. This could range from the vile to the ridiculous. On I Love Lucy, the source of trouble is Lucy's own crazy schemes, while on Dexter, the bad guy has to be someone so loathsome and dangerous that a vigilante serial killer looks good by comparison. Superheros need supervillains. Vampire slayers need vampires.
On House, the doctor always faces problems on three levels: the disease (a medical mystery), the fact that patients always lie about some crucial detail needed to solve the mystery, and his own irascible behavior which gets him in trouble with authorities, his friends and his own health. Individual episodes have their own themes, but the unifying is that "everybody lies," mostly for self-serving purposes, but sometimes out of concern for others, and that reason and science needs to uncover the psychological and social truth as well as the physiological and medical truth to cure the disorder.