The Doom Slayer and Charlie would be an interesting pair. Charlie is all about redemption and trying to make Hell a better place, while the Doom Slayer is focused on pure destruction of demons. Their meeting could lead to some great character development. Charlie might try to convince the Doom Slayer that not all demons are irredeemable, while the Doom Slayer could show Charlie the true horror that some demons are capable of.
Alastor and the Doom Slayer would also be a fascinating combination. Alastor is a powerful and somewhat sadistic demon in Hazbin Hotel. When he meets the Doom Slayer, who is feared even among the most powerful demons in his own universe, there would be a power struggle of sorts. Alastor might be intrigued by the Doom Slayer's strength and try to use him for his own ends, while the Doom Slayer would see Alastor as just another target.
Vaggie and the Doom Slayer could be an unexpected but interesting pair. Vaggie is protective of Charlie and her vision for Hazbin Hotel. When the Doom Slayer arrives, she would be very suspicious of him. However, as they face common threats in Hell, they might learn to work together. Vaggie could provide the Doom Slayer with some knowledge about the unique aspects of Hazbin Hotel's Hell, and the Doom Slayer could offer his combat expertise.
Maybe the Hazbin Hotel is in the middle of a crisis where the souls of the damned are being consumed in a strange and new way. Charlie and her crew suspect it has something to do with a portal that has opened up to a dimension filled with the kind of demons the Doom Slayer is used to fighting. They seek out the Doom Slayer, who has to navigate the politics and oddities of Hazbin Hotel's Hell while trying to close the portal and stop the consumption of souls.
Another interesting interaction could be between Vaggie and the Doomguy. Vaggie is protective of Charlie and the hotel's mission. When she meets Doomguy, she might be suspicious of his motives. She could be constantly on guard around him, and he might be a bit annoyed by her over - protectiveness. Their relationship could start off as very tense but could develop in interesting ways as they get to know each other better.
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 possible plot could be that the Doom Slayer somehow ends up in the Hazbin Hotel universe. He might initially be seen as a threat due to his violent nature. But then, they realize he could be an asset in dealing with some of the more dangerous demons in the Hazbin Hotel world. For example, he could help Charlie and her crew deal with some of the more unruly and powerful demons that are causing trouble in and around the hotel.
Charlie could be a great character to pair with Lucifer. Their opposing views on the redemption of sinners in Hell can create a lot of interesting tension and storylines. Alastor is also a good choice. His power and unpredictable nature can make for some really exciting interactions with Lucifer. And then there's Vaggie. Her protective nature towards Charlie can lead to some great conflict when she has to deal with Lucifer.
I think Niffty would be an interesting character to pair with Alastor. Niffty is always cleaning and being hyper - active, and Alastor could either find this endearing or extremely annoying. Their dynamic could be quite comical. Also, Husk could be paired with Alastor. Husk is a grumpy alcoholic and Alastor's attempts to either annoy him further or get him to do something useful could lead to some engaging storylines.
Charlie would be a great character to pair with Adam. Charlie is all about redemption and Adam could either be a foil to her or be influenced by her positive attitude. Their interactions could be really interesting as they have different outlooks on the whole idea of sin and redemption in the Hazbin Hotel world.
One possible plot could be that the Doom Slayer somehow gets teleported to the Hazbin Hotel universe. He then has to deal with the demons there, which are different from the ones he usually fights. The hotel staff might initially be afraid of him, but then realize he could be an asset in dealing with more malevolent demons.
There are a few out there. However, the quality can vary a great deal. Some fanfictions might be more action - oriented, with the Doom Slayer getting involved in the chaotic world of Hazbin Hotel, perhaps fighting against some of the demons there in his own brutal style. Others could be more about character relationships and how the two very different characters from different franchises would get along or clash.
One interesting concept could be crossovers with other popular animated shows. For example, a crossover with 'Steven Universe' might bring unique character interactions. The different power sets and personalities from both universes could create engaging storylines. Another concept could be a crossover with a classic fairy - tale world. The demons from Hazbin Hotel interacting with fairytale characters would be quite a sight, like the sinners trying to corrupt the pure - hearted fairytale folks or vice versa.