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 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.
In Hazbin Hotel Charlie x OC fanfiction, the OC can bring fresh perspectives. Charlie is often associated with her mission to redeem sinners in Hell. An OC might challenge this idea or support it in unexpected ways. The OC could be a demon who has a more cynical view of redemption, which could create tension. On the other hand, they could also be an angel - like figure in Hell (even if that's an oxymoron in the context) who helps Charlie see new ways to approach her goal. Moreover, the fanfiction can play with different art styles if it's accompanied by illustrations. Some may depict Charlie and the OC in a more cartoonish style similar to the original Hazbin Hotel art, while others might go for a darker, more detailed aesthetic.
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.
One possibility could be that in the fanfiction, Charlie's journey to Heaven is a test of her redemption ideology. Maybe she goes there to prove that even sinners from Hell can have a chance at salvation. She might encounter angels who are skeptical at first but are gradually won over by her unwavering belief in the goodness of all beings.
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.
Redemption is a common theme. Charlie is all about redeeming sinners in Hell, and an OC can either be a part of that mission or a hindrance. Another theme is self - discovery. The OC, through their interactions with Charlie, may learn more about themselves, their place in Hell, and their own potential for change. And of course, friendship or love can be a theme too, as they build a connection with Charlie.
One popular Hazbin Hotel Charlie x Alastor fanfiction is 'A New Alliance'. It explores their relationship as they team up to deal with a new threat in Hell. The story delves into their different personalities, with Charlie's optimism clashing and also meshing in interesting ways with Alastor's more menacing nature.
The first meet in the fanfiction could be a chance encounter. Charlie might be wandering around the hotel, thinking about how to make it better. Vaggie could be on the lookout for any threats. When they meet, Charlie's friendly approach might surprise Vaggie. Charlie starts talking about her dreams for the hotel and the sinners' redemption. Vaggie listens, and slowly, she begins to see the possibility in Charlie's plan. This first meeting is the spark that begins their journey together in the Hazbin Hotel story.
Another theme is growth. Fanfictions often show Charlie and Vaggie growing both individually and as a couple. For example, Charlie might learn to be more assertive in her relationship while Vaggie might become more open - minded, and these changes are usually related to the challenges they face together in running the hotel or dealing with other demons.