Would immortals reproduce?
I've been scribbling lots of notes about Queen of Revels, which might be a novel someday if I ever get a callback from a certain editor.

Again, it's the "immortal person follows the historical trends that leads to BDSM" idea.

Now, I do not want to do this person as a vampire. I think vampires are rather overexposed, particularly in the urban fantasy/erotic fantasy genre, and the trope has been, as it were, sucked dry. Plus, there are logistical difficulties of how influential a person in the past could be if they couldn't go around in daylight. Remember, most people in the past lived with the sun. There was no night life, really. Furthermore, when you do a vampire story you have to do your variation of "the list": crosses, no; garlic, yes; sunlight, yes; running water, no: etc.... It's tedious to do that. It diminishes the sense of wonder and/or horror if the creature is so well known.

So, non-vampiric immortals. I've been making notes about their psychology (how many lifetimes-worth of memories can a human brain hold?), their society (if you don't really need others of your kind, even for reproduction, why have a society at all?) and their physiology (Does inhaled dust accumulate in lungs over multiple lifetimes?)

The only point I can't make up my mind about is whether they are infertile. In the Highlander franchise, it's a big point that immortals, male or female, can't have children, but I don't see why. Naturally, a freely reproducing immortal species of humanity would rapidly overpopulate the world. That would easily be solved by saying that they can sire or bear children, but they are normal humans. Human males produce millions of sperm cells, and human females produce tens of thousands of eggs.

Furthermore, I want to take a relatively realistic take on immortals, so they are not completely unkillable. There could be a very slow attrition over the millenia, despite superhuman regenerative powers and lots of experience in avoiding danger. There could be a need for however infrequent reproduction, withere by nature or by design.

On the other hand, whether they are natural or artificial beings, immortals don't need to reproduce. They would have normal human sexual and reproductive functions, in order to blend in with mortals, but shooting blanks. (I want to have a joke about a female immortal complaining about 12,000 years of periods.)

It's hard to say which is worse: never having children of your own, or having to leave them to hide your secret, or sticking around and watching them get old and die? Immortality is bittersweet enough without getting angsty about drinking blood or never seeing sunlight.

It isn't a big part of the story as I envision. Either way, the story is pretty much the same. However, it is a question that will appear in the reader's mind, and to not give an answer would be bad faith.

The other issue is fitting the genre, since the editorial nibble is from an online romance/erotica publishing company. My view of the story tends toward some action scenes, cranked up to a ridiculous degree because the people involved consider chainsaws to be foreplay, but the publisher may not want that, or that much. They'll also want lots of romance, which is a new thing for me. In the first submitted draft of "The Innocent's Progress", it ended on a down note, with a potential romance still born. It took a major effort for me to do another draft with an ending that hinted they might get together.