As I pointed out in my earlier post, Nightmare was the original 'plan' for the father. There were then other plans that never came together.
Uncanny X-Men Annual #4 (1980), Kurt celebrates his 21th birthday. Margali Szardos uses the Eye of Agamotto from Dr. Strange and the images and captions reveal:
“…the infant Kurt Wagner — barely an hour old — found beside his dying mother, taken in by the Gypsy Witch-Queen Margali Szardos, and raised as one of her own.”
Uncanny X-Men #142 (1981), Nightcrawler met Mystique, and noted she had a physical resemblance to him. her “Who are you?!” he asked and she replies “Ask… your mother, Margali Szardos. Who would better know than — she?”.
Uncanny X-Men #170 (Jun 83), captions for a dream sequence where Mystique is getting hunted by Lady Jean Grey and Sir Jason Wyngarde, note it to be occurring in: “1783, the place England, their quarry a woman who will not be born for another 170 years”, Meaning Mystique was born in 1953,
30 years before the story’s original setting.
Along the way, CC brought in the demon-sorcerer of Limbo, Belasco, as a foil for the X-Men. Belasco had pointy ears like Nightmare, ruled a demonic dimension(Limbo), and even had a forked, prehensile tail just like Nightcrawler. Belasco made a deal with his “Elder Gods”(the N’Garai) for immortality on the basis he would return to earth and become the “Father” of a new race of Earth-Born Demons. In the Magik Limited series from 1983–1984, Kurt was the X-Man who fell most under Belasco’s control, even becoming his “familiar”. It seemed their might be a connection between them...
Uncanny X-Men #177 (Jan 84) Kurt is beginning to question Margali’s account of him as a foundling. When he asks his girlfriend Amanda Sefton“Who am I…? Where do I come from?! What is my real family?” she responds “I know what Mom told me — she found you, new-born and barely alive, in a roadside shelter in the Black Forest. A man — your father, I guess — lay outside…”
Also in Uncanny X-Men #177, Mystique kills six of the seven X-Men robot sims of the X-Men practicing for an attempt to get Rogue back to her and Irene, she hesitates when it comes to the robot Nightcrawler. Destiny says: “You could not harm a facsimile Nightcrawler — how will you fare against the man himself? If he’s killed…” Mystique then responds “Be silent, woman! Mention him again… at your peril. The X-Men have my child and if I have to slaughter them all to rescue her, then I shall!”
Uncanny X-Men #204 (1986), which introduced Judith Rassendyll, the last of the Elfburgs, and was supposed to lead to his parentage. But Claremont couldn't get the story to come together to his liking(called it a 'dud').
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What can we gather from all that? That Margali Szardos lied to Amanda for some reason when she said she found Kurt next to his dying
father, that it was his mother instead(Eye of Agamotto doesn't lie). That Margali Szardos and Mystique knew each other. That there was a familial relationship between Kurt and Raven. That Mystique was having to prepare herself for the possibility of killing someone she cared something about(Kurt) to retrieve someone she regarded as her child(Rogue). That Mystique was then around 30 years old when she met Kurt, and he around 20.
Given all that, you can likely see that the familial relationship CC was intending at that point was brother and sister, with Mystique the older sibling to Kurt. There is no way he was setting Mystique's age at thirty, Kurt's at twenty and intending her to be either of Nightcrawler's parents. He was also clearly setting up Belasco as Nightcrawler's father, along the same lines he had planned for Nightmare and with Kurt as one of his 'Earth-Born Demons'. Having Mystique not only be Kurt's sister but Belasco's child as well also explained some her demonic imagery(the skulls). That was
the plan as of the mid/late 80's. Mystique his sister and Belasco their shared father.
Most fans had pegged Belasco as his dad and liked the idea, and the dangling question of his connection with Mystique could have been easily resolved with the sister revelation. Instead we got a muddled mess.
Claremont didn't hit upon the idea of retconning Mystique to be much older than previously depicted, able to shapeshift to the point of producing sperm and conceiving a child with a geriatric until he was writing Excalibur. Then a 64 page Excalibur hardcover graphic novel was announced to ship in December 1990: "Chris Claremont and Alan Davis continue their Excalibur collaboration with the biography of Kurt Wagner Nightcrawler, from his birth to his rescue at the hands of Charles Xavier. We will finally learn more of the mysterious connection between Nightcrawler and Mystique!"
At that point Shooter was a few years gone as EIC, the planned graphic novel would have been released without the Comics Code, and it was only about a year later that Northstar came out, Code-be damned.
As I said, Claremont gave up on it because he couldn't make it work. And this story doesn't work either.