Another option is a crossover with a superhero universe. Imagine Big Boss in a world with the X - Men. He could be seen as a mutant because of his enhanced abilities. There could be a lot of conflict and cooperation as he tries to find his place in this new and strange world.
Maybe a crossover with a post - apocalyptic character. Big Boss could find himself in a world like The Last of Us and have to deal with the infected and other survivors. His military training would come in handy, and it would be interesting to see how he adapts to that different kind of threat.
Maybe a crossover with a post - apocalyptic world. Big Boss enters a world like that of The Walking Dead. He has to use his survival and combat skills in a completely different scenario. He could form an alliance with some of the survivors there, teaching them his ways of combat and in return, learning about their unique ways of surviving in a world full of zombies. It would be a great mix of two different universes.
One idea could be a crossover with Hazbin Hotel. The characters from Helluva Boss could interact with those from Hazbin in the chaotic afterlife setting. For example, Blitzo could try to hire some of the Hazbin characters for a job at I.M.P. Another idea is a crossover with a more mainstream series like Adventure Time. Picture Moxxie and Millie exploring the Land of Ooo and having to deal with its wacky inhabitants while also trying to complete a hit. Or perhaps a crossover with My Little Pony. The contrast between the dark and raunchy world of Helluva Boss and the colorful, friendship - based world of My Little Pony could create some really interesting and comical scenarios.
One idea could be a crossover with 'Toy Story'. The Boss Baby could end up in the world of toys and have to deal with Woody and Buzz. Maybe he tries to take over the toy hierarchy, thinking he can run things better. Another could be a crossover with 'The Incredibles'. The Boss Baby might try to recruit the Incredibles for some top - secret baby - related mission. He could use their superpowers to achieve his goals.
You could do a crossover within different fable universes. For instance, take the moral - driven characters from Aesop's fables and put them in the world of Native American fables. The animals in Aesop's fables might learn new values from the Native American fable characters. Also, consider mixing fables from different cultures, like Asian fables and European fables, in a fanfic.
Maybe a crossover with a fantasy world where the Darkness entities interact with magical creatures. For example, they could form alliances or rivalries with elves or wizards, which would lead to a whole new set of adventures and conflicts.
A uzumaki crossover fanfic could involve crossing over with 'Bleach'. Picture an Uzumaki character in the Soul Society, their strong chakra being mistaken for reiatsu. They could bring new fighting styles and techniques to the Bleach universe.
One idea could be a crossover with 'Naruto'. Gon could meet Naruto and they could have a friendly competition in terms of strength and speed. Another idea is a crossover with 'One Piece', where the Hunters from 'HxH' join the Straw Hat Pirates on an adventure. Maybe they could be searching for a rare treasure that has both the allure of the Grand Line and the mystery of the 'HxH' world.
One idea could be a Dalek crossover with the Doctor Who companions. Imagine the Daleks facing off against a group of companions who have to use their unique skills and knowledge to outwit the Daleks. Another could be a crossover with a science - fiction adventure like Star Trek. The Daleks' aggressive nature would contrast greatly with the more diplomatic Starfleet.
One idea could be a crossover with 'Friends'. Jerry could visit Central Perk and have a stand - up comedy session there. His unique sense of humor might either amuse or confuse the 'Friends' gang.
One idea could be a crossover with another popular RPG like Persona. The characters from Twewy could enter the Metaverse in Persona and interact with the Phantom Thieves. They could learn about the different ways of fighting Shadows compared to the Noise in Twewy.