As I explained before, Gifts pretty much said everything I wanted to say about my four changeling characters. Thus, initially, I considered the novel a complete, standalone product. Since my primary interest is in SF with a political or police-procedural bent, having completed my first Urban Fantasy was something of a relief.
It's not that I didn't enjoy writing the book. I got to know and respect my characters; some of their voices I'll probably be hearing in my sleep for years to come. Creating a system of magic, however, and maintaining a semblance of verisimilitude in descriptions of that system is something of a pain in the ass. It's rather like creating a new branch of science, complete with axioms, laws, and theorems. Just the simple act of keeping in mind which principles are inviolable and which are merely traditional becomes something of a headache. I solved a large set of problems by simply postulating that my system of magic would not violate the First Law of Thermodynamics. Of course, most magic in epic fantasy is designed precisely to allow violating that Law, so I was, in a sense, making things hard for myself. It's not all that difficult to manage, though. It's just a matter of controlling the scope of the system in any given conjuration. Of course, that's only one aspect of the magical system I devised. I also had to specify—off-stage, so to speak—classifications of magic, sources of power, methods of spell casting. I had to decide whether names like mage, sorcerer, witch, wizard, and thaumaturge had any caste significance (they don't) and how I would differentiate between conjuration, manifestation, transformation, glamour, illusion and how or in what manner I would invoke clairvoyance, clairaudience, prescience, and telepathy.
Ultimately, it was that system of magic (and two other lacunae I'll address in a moment) that convinced me I still had a wealth of story-potential begging to be tapped. Before I could get the continuation of Gifts corralled into some semblance of plot, I had to come up with a framework. The system of magic provided that framework. Actually, it was a combination of the system of magic and the politics of the collective of mages (which—both the politics and the collective—I only hinted at in Gifts) that gave me the outline for the series, which I currently plan to include Talents and a third book yet to be named. In Gifts, one of the first claims Melchior makes is that no written record exists describing the Supernal Fyrd. No histories; no biographies; no directory of mages; no books of spells, runes, incantations; no compendia of rules, regulations, laws, traditions, best practices—not even a (much needed) glossary of magical terms. Now, while it's easy to justify such a tradition, it's damned near impossible to police it. Nor, in a world of 10,000 mages hiding among seven billion mundanes, is consensus on such a practice likely. Zane, the antagonist of Gifts, says he doesn't consider himself a member of the Fyrd nor subject to their legislation. Surely, I thought, others would share this attitude. I began compiling a likely structure of the politics of the Fyrd, and the plot followed.
As for those lacunae, first, I wanted to explore at least one case of the opposite transformation: one female-to-male transition. The patriarchal hegemony is, sadly, alive and—eh, I wouldn't say it's "well," but it isn't showing many signs of dying. The vast majority of US Senators are male. We still haven't elected a woman president or VP, and the average woman in the US still only makes $0.79 for every dollar her male counterpart makes. Sexism alone might be enough to make remaining male worthwhile for a woman magically transformed to male. Of course, the transformation is still difficult, energy intensive, and would likely require a loss of life to accomplish. All good reasons not to try reversing such a change.
The other lacuna I meant to address in Gifts (it just never seemed to fit anywhere) is a matter of how people in a supposedly-scientific, civil society react to magic. I think magic could easily be all around us, and that we ourselves would provide the majority of the propaganda of disbelief as a matter of course. If, for example, a man were standing on a sidewalk in the downtown area of any major city and suddenly collapsed into a pile of serpents, which then wriggled out of his clothing and writhed en masse into the gutter and down a nearby storm drain, how do you think onlookers would respond? I believe they'd applaud. Even if several people videoed the entire event on their smart phones, pundits, panelists, entertainers, and "experts" would spend the next day or so dissecting the event on the news. Every unexplained glint of light would be proof of mirrors. Speculation would abound that this event was advertising for a coming movie, TV special, or broadway extravaganza. Holographic projection and such names as Chris Angel, David Copperfield, and Penn & Teller would be bandied about. Do you seriously believe anyone would openly say, "I think it's a magical transformation"? Of course not. Everyone knows such a thing is impossible. Denial is a powerful weapon, and we tend to wield it against ourselves.