A plot could involve Lucifer and Adam teaming up to right a wrong in the cosmos. Maybe there's a force that threatens both heaven and the newly created earth, and their combined knowledge and powers are needed. Adam brings his connection to the earth and its new life, while Lucifer has his own unique powers and understanding of the celestial realm.
One interesting plot idea could be that Lucifer, seeing Adam's pure and innocent nature, decides to test him in various ways. He might create illusions of worldly temptations to see if Adam can resist. Another could be that Adam, being curious about the fallen angel, tries to understand Lucifer's fall from grace and in the process, they form an unlikely friendship that challenges the norms of heaven and earth.
One plot idea could be Lucifer seeing a different side of Adam that no one else has. Maybe Adam shows kindness towards a lower - class demon, which makes Lucifer question his own views. Another could be that they team up to solve a mystery in Hell that threatens both the sinners and the native demons. And a third idea is that Adam tries to introduce Lucifer to more 'human' concepts like forgiveness, which Lucifer initially resists but gradually comes to understand.
One idea could be that Lucifer has a secret mission on Earth related to the events in Highschool DxD. He could be observing the main characters to see how they are handling the supernatural situations, and perhaps he has a greater plan in store for them. Maybe he's looking for a new heir or a special power that could be hidden among the students.
One idea could be exploring Adam's adventures in an alternate universe. Maybe in this universe, he has to team up with a different version of himself to save the day. Another could be delving into his relationship with his mother before the events of the movie in a more in - depth way.
One idea could be that Harry, with his magical skills, somehow ends up in Lucifer's realm. He tries to find a way back home while Lucifer is intrigued by his magic and wants to learn from him. Another could be that they team up to fight a common enemy that threatens both the wizarding world and Lucifer's domain.
One idea could be that the baby has some sort of special power. Maybe it can see things that others can't, like angels or demons in their true forms. This would add an interesting dynamic as Lucifer and Chloe try to understand and protect their child from the otherworldly forces that might be interested in the baby's power.
One idea could be that they form an alliance to rule over a certain realm. They combine their powers and knowledge to bring order in a chaotic land. Alastor's cunning and Lucifer's authority could make for a very interesting power dynamic.
One idea could be a new character joining the school who has a mysterious power that both the devils and angels are interested in. This new character could disrupt the existing power balance in the school.
One idea could be Percy having to struggle with the dark reputation that comes with being Lucifer's son while trying to prove he's not all bad. Maybe he forms an unlikely alliance with some angels who initially distrust him but later see his good side.
One idea could be exploring Lucifer's past before the events of Hazbin Hotel. Maybe show his rise to power and the decisions that led him to become the ruler of Hell. Another could be a story where Lucifer has to deal with a new, powerful demon who challenges his authority in Hell. A third idea might be a fanfiction that focuses on Lucifer's relationships with the other main characters in a more in - depth way, like his true feelings towards Charlie and how he views her plan for the hotel.
Maybe the pregnancy is unexpected, and they initially panic. But then they start to get excited about building a family together. Chloe could start to train Lucifer on how to be a good father, like teaching him about baby care and normal family values, which would be a big change for him since he's from Hell.