This is NOT up for adoption.
As Danny Fenton grew older, he met many challenges in his life. They included rematches with ghosts he'd already fought, conflicts with Vlad with results that haven't changed, rescuing his human friends from ghosts and even, on rare occasions, rescuing ghost friends from humans.
Danny, Sam, and Tucker all went to college; different colleges that were all near each other. As a freshman, Tucker became a focus of attention for Technus. Tucker learned the basics of ghost technology from Danny's parents and then used that to invent means of fending the ghost off. After a series of incidents, they became mutual friends and would often work together on technology both ghostly and not. This news came as a great surprise to Sam and Danny for neither could have predicted a partnership between the two.
Sam became an art and literature student. Due to her association with Danny, 'the Halfa' occasional ghost artists would come to her to discuss, argue, create, or all simultaneously.
Danny continued to grow in his powers, though once in college the ghostly part of his life seemed to level out. He finally soundly defeated Vlad Masters and Vlad was forced to recognize that he could no longer harass the younger halfa. They developed a civil relationship, though no one would go so far as to call them friends. In college, Danny also met scholarly ghosts who had merely academic interests and wanted only to teach, not to fight. These ghostly professors, as well as the living ones, encouraged Danny's desire to learn, which he had much more time to do as he could now easily manage the ghostly part of his life. He discovered a great interest in anthropology and graduated to spend his post-college life traveling around the world experiencing and learning of many other cultures.
Though they did grow apart, Danny, Sam, and Tucker often would arrange time to be together and they stayed good friends for a long time. It was at one of these meetings, many years after the initial portal incident, that something rather intrinsic was pointed out to Danny, rather unintentionally by Sam. The day was just after Danny's recent return from South America. It was autumn and the three were meeting at Tucker's home.
Danny, who had not seen his friends' faces in several months, was just taking them in, enjoying being in their familiar presence. Tucker's face had started to take on lines of age, Sam's more so, though she would disguise them with some fashion of makeup or other. Tucker wore lounge clothing that, due to his comportment, still made him seem like a professional in some field. Sam was wearing a pair of worn, paint-stained overalls and a black shirt.
"It's good to see you again, Danny. Where have you been off to adventuring this time?" Sam initiated the conversation as she sipped at her tea.
"Southern Nicaraugua. What about you? And Tucker?"
"I've been to Europe to study the art in gothic architecture, but I came back a few months ago."
"Technus and I have been working on inter-dimensional communication devices."
"So we could speak from here to the Ghost Zone?"
"Yep!"
"You know, Danny," hesitation colored Sam's voice, "years have passed since we all graduated, but you still look young. I'm nearly envious."
The conversation between the three friends continued on from there, touching many different subjects as they spoke throughout the day, but Sam's comment stayed with Danny. It struck him as odd, enough for him to go back and look at a picture of himself from just after college, a picture of all three of them together celebrating their graduations. Danny immediately noticed how time had touched his friends, but when he looks in a mirror and compares his face from then and now, he does not see any significant signs of aging. There are some newer scars from his travels, but there aren't really wrinkles or age spots or anything of that sort. Almost as if he weren't physically aging.
Is it because I'm half ghost? Ghosts don't physically age, though they can change their appearances. Danny discovered how to change his appearance a few years ago and decided to change his jumpsuit to something a bit more normal-looking. He can pass off as a normal human while in ghost mode if he focuses on doing so.
A week later, Danny visited Vlad to discuss the idea of aging only to find that Vlad, too has visibly aged, albeit much slower than a regular human. During the visit, the billionaire also comments on Danny's seemingly unchanged appearance.
With no one else Danny can think of for guidance, he decides to visit Clockwork, his ghostly self-appointed guardian.
"It is true, young Danny, that a half-ghost such as yourself or Vlad, will not physically age at the same rate a human would."
"But even Vlad has aged more than I. Is it because I was exposed to more energy than him initially?"
"That is also a factor, but it is not the important one."
Danny waited silently.
"You see, young one, all ghosts who have an official guardian will take on some of the attributes of that guardian."
"Like an apprentice?"
"Of sorts... However, even that is not the real cause. If you were merely my apprentice, you would only ever achieve a small control over time, after a great deal of practice. You would be able to slow or accelerate time around yourself for short periods and resist time's toll on you."
"So, what is it really, then?" Danny was curious about having some small control over time, but he wanted to know what Clockwork was getting at.
"You are my heir, young Danny, and will eventually take my place."
Danny, for the first time in years, gaped, unable to speak.
Somewhere, a quiet voice in Danny's head said, I'm Clockwork's Heir?