The popularity might be due to the complex and interesting characters of Vox and Valentino. Their personalities in the show are full of mystery and charm, which gives fanfiction writers a lot to work with.
One interesting aspect of Valentino fanfics could be his business dealings in Hell. Maybe a fanfic shows him making a new deal with a rival or trying to expand his empire. Another idea could be a fanfic where Valentino has a change of heart, which would be quite a twist considering his character.
I'm not sure of a specific one off - hand, but you can start by looking on fanfic platforms like Archive of Our Own. There are usually tags for Valentino and Hazbin Hotel that can help you narrow down your search. Just look for ones with high ratings and good reviews.
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.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Hazbin Hotel. Just use the search function and type in 'Vox x Valentino' to find relevant stories.
The relationship could be developed through shared goals. For example, if they both want to expand their influence in Hell, they might start by making small deals with each other. Then, as they see that they can benefit from working together, their relationship could grow. Another way is through conflict. Maybe they start by hating each other, but as they face challenges together, they start to respect each other more.
One common theme is power dynamics. Alastor being a powerful figure in Hell, when paired with an OC, often leads to exploration of how their different levels of power or influence interact. For example, the OC might be initially intimidated by Alastor's power but then find ways to hold their own.
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 plot idea could be that Valentino and Vox form an unexpected alliance to take over a new part of Hell. They might start by manipulating some of the lower - class demons to do their bidding. Another could be a story where they both have a falling out over a shared interest, like a powerful artifact in Hell, and then compete against each other in a series of challenges. A third idea is that they have to team up to face a common enemy, perhaps a new demon who is threatening their power and status in Hell.
One popular 'hazbin hotel alastor x oc fanfic' is 'A Meeting in Hell'. It has an interesting plot where the OC is a new arrival in Hell and catches Alastor's attention in a very unique way. The author does a great job of portraying Alastor's character as we know it from the show while also developing the OC's backstory and personality in a way that makes their interactions engaging.