One idea could be Naruto somehow ending up in the Hazbin Hotel universe. Maybe he's on a strange mission and gets transported there. He could use his ninja skills to help the characters in the hotel deal with some of their problems, like fighting off demons that are causing trouble outside the hotel's protection. Another plot could involve the Hazbin Hotel characters visiting Naruto's world and being amazed by the different kind of powers and cultures there.
A really cool plot could be that a powerful enemy threatens both the Naruto world and the Hazbin Hotel universe. So, Naruto teams up with the Hazbin Hotel crew. The Hazbin characters could use their charm and wits, while Naruto uses his combat skills. They would have to learn to work together despite their differences. For example, Charlie might try to reform some of the more evil foes using her rehabilitation ideas, while Naruto takes them on in a more physical fight when needed.
One idea could be that God decides to visit the Hazbin Hotel incognito to see if redemption is truly possible in this unique afterlife setting. He could interact with the characters like Charlie and Vaggie, observing their efforts and maybe secretly guiding them in subtle ways.
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 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 that the characters from Hazbin Hotel end up in the wizarding world of Harry Potter. Charlie could try to reform some of the dark wizards using her methods from the hotel. For example, she might see Voldemort as a 'lost soul' and attempt to rehabilitate him, which would create a really unique dynamic as the other Hogwarts characters react to her unorthodox approach.
One idea could be that some of the Hogwarts students accidentally end up in Hell and have to find their way back with the help of the Hazbin Hotel crew. Maybe they could exchange magic knowledge for help in navigating the strange and dangerous underworld.
One idea could be that Adam and Eve end up in Hell and have to adapt to the chaotic and sinful environment of Hazbin Hotel. They could meet the main characters and get involved in their schemes to rehabilitate sinners.