You could also write a fanfiction about a new character arriving at the Hazbin Hotel. This new character could have a unique power or a very different personality from the existing ones, which would create all sorts of new interactions and storylines. Maybe they have a special ability to see the pasts of the other characters and use that to help them on their path to redemption.
One idea could be exploring the backstories of the characters in more depth. For example, delving into how Alastor became the Radio Demon. Another could be creating a new character who arrives at the hotel and causes a stir among the existing residents. Maybe a character from a different dimension or with a very unique power.
One idea could be exploring Lucifer's past before ruling Hell in the Hazbin Hotel universe. Maybe his rise to power, his relationships with other powerful entities at that time.
Another option is a crossover with Gravity Falls. Dipper and Mabel could end up in Hell for some wild reason, like following a strange portal. They'd meet the Hazbin Hotel crew and chaos would ensue. Dipper would be all about trying to figure out the logic and secrets of this new place, while Mabel would probably befriend some of the more outlandish demons or sinners. Charlie would see the Pines twins as a chance to prove that redemption is possible even for those from other dimensions.
One idea could be Adam's backstory exploration. Maybe he had a different life before coming to the Hazbin Hotel universe and that could be delved into. For example, his family, his upbringing, and how those things might have shaped his current self.
One idea could be an OC who is a fallen angel trying to redeem themselves in the Hazbin Hotel. They could face challenges from both the sinners and the hotel staff, adding an interesting dynamic.
One idea could be that God decides to visit the Hazbin Hotel to see if there's any hope for redemption among the demons. He could interact with Charlie and the other characters, causing them to question their beliefs and actions. Another could be a story where God sends an angel to the hotel undercover to observe and report back, and the angel ends up befriending some of the demons, blurring the lines between good and evil.
Well, a plot idea could be that Alastor discovers a new power within himself in the context of a fanfiction. This new power could either help or hinder his standing in Hell. Also, perhaps a plot where Alastor tries to start his own 'radio show' in Hell that's different from his usual broadcasts, and it causes all kinds of uproar among the demons. Or, what if Alastor is forced to team up with someone he really dislikes, like Vox, to face a common threat to their power in Hell?
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.
You could write a fanfic about a competition among the demons in the Hazbin Hotel. They could compete in various challenges that test their skills, whether it's singing, dancing, or something more devious. The winner could get a special prize, like a one - day pass to visit the human world or a chance to be upgraded to a better room in the hotel. This would show different sides of the characters and create a lot of interesting interactions.
One idea could be a human who ended up in Hell due to a deal gone wrong with a demon on Earth. They have to navigate the strange and dangerous world of Hazbin Hotel while trying to find a way to reverse their fate. Maybe they team up with some of the hotel's residents who see an opportunity in their unique situation.