This idea is mainly how to fit in the demigods into the world building for Marvel or Harry Potter.
For both other series, other species are almost always magical or not, with no imbetween. Especially plants and animals. So we take that idea, and say all humans are magical. Those not strong enough to use their own power, unconsciously merge theirs with others with similar beliefs.
As opinion helps shape magic, polytheistic people without enough power create their own gods, as well as their supernatural enemies that demigods must take care of, for the most part.
There's a minimum amount of knowledge taken from the original population to other cultures required in order to tie in that religion to one of the flames of humanity (Western, Eastern, etc.), and therefore, all of humanity. A pantheon needs to be tied in once their original people aren't large enough to maintain the energy that made them. Currently, the Greco-Roman Pantheon has the strongest Percentage of a flame. The current least known that had enough of a tie in order to resist fading after their peoples demise is the Aztecs.
For Marvel, The Asgardian pantheon are both tied into a flame, and Aliens. They are considered a parasitic race by the other pantheons, as they made themselves be seen as gods in 7 different planets (9 realms, 7 hosts, them, and Jontons, their enemies) for those species to give power to the Asgardian people. With that power, they are supposed to take responsibility for the 7 subjegated planets. As they left long ago, most pantheons see them as only coming back after they perverted their tie to the Western flame so much it began to break. Others see them as having given up all claim by leaving, and now view them as trespassers.
For Marvel, this storyline concludes in my head as the arc of Thor regaining his ties to the Western flame, and maybe repairing his personal relationship with peer pantheons, becoming fully supernatural rather than alien. He rejects the the throne of Asgard to do so after realizing what they've done. Loki abandoned Asgard long ago, to become fully supernatural. Many of the nature gods separated from the main pantheon as well to oversee their duty. The Jontons mainly just like that Asgard isn't as powerful without them, and are indifferent to humanity. Loki had taken sanctuary with the other pantheons before he was mind controlled, but as it was a mortal affair the gods couldn't interfere directly.
Loki actually came to his senses when he was captured, and with the help of Hermes, escaped and used his tie to the Western flame to fool the Asgardians into thinking he was in custody. This snapped the last of his ties to the aliens, making him fully supernatural. Loki met Percy when visiting his son the world serpent for the first time in centuries, as he had been forced from the company of all his children previously. Jormander had also sought sanctuary, and resides within Poseidon's kingdom as an honorary brother to Triton, who adopted him as a sibling.
For the Harry Potter universe, I like to keep their ages about the same by having Harry be Gen Z or a Millennial. In this, no life is without magic. Those without the power to mould their own magic, unconsciously contribute to a single flame of humanity, which brings all polytheistic deities and species to life. Those deities give back power in turn for their creation with either offspring or other forms of power. Muggleborns are a result of Isis, Norse magical gods, and Hectate.
Usually the more personal gifts, are the more caring deities. Children as the way to give back power and renew the ties to the flames of humanity after the original beliefs died out, for the more indifferent or uncaring deities. Some, like Poseidon and Thanatos and Hades, do both. Mostly because of a genuine care for humanity, and not seeing themselves as superior due to species allowing them to connect with individuals rather than in concept only.
Their contributions are useful and cute aquatic magical species(Tyson did not get his love of all creatures from his mother), the deathly hallows, and death magics(good ones, not the type that defy the nature of death as abominations like horuxes, the most common true death magic being allowed to connect passed on loved ones on Sanheim with rituals, and allowing souls to stay after passing if they choose for a specific purpose. (It's an allowed trade, Hades justifies it as delagating some of his responsibilities to humanity but really is a softy who hates putting anyone through being without a loved on until a specific time like he has to with Persephone. Many times he makes up ghostly duties or responsibilities he doesn't have to allow them to stay.)
Most wizards forget that death is a natural element like water and fire, and consider all death magics abominations, which actual abominations wouldn't be if death wasn't natural. It has to be natural to pervert the natural order. Because of them being unable to see the line, Hades has to send Nico or an employee like Thanatos if something perverts his gifts, as true dark wizards know the law is unjust, but don't know where the line actually is and will protect the evil along with their normal dark peers. Which is how Nico ends up going to Hogwarts. The de Angelo Line has actually been connected to the makers of the true dark magics for generations. They aren't legacies because their scions with divine beings usually don't have children. However, they were all blessed, but all but Maria de Angelo and her children were killed by Grindelwald searching for the deathly hallows. (One of their members had given the hallows to the brothers on Thanatos's order, as the Perevells had earned his appreciation for helping with a problem, but he was too busy dealing with other problems to do so in person.) Grindelwald did not get the information he sought, because all the De Angelo's would die in the name of Death, the being and related beings. Nico's instinctive loyalty and sense of duty to the natural order of death came as strongly, if not stronger, from his mother rather than his father.
And that wraps up my ideas of Demigods in Marvel and the Harry Potter Universe without altering too much of either world building. Divine beings ancient laws are really just practices that either keeps one from damaging their ties to humanities magic, or reinforces the connection.
