Toriel would also be an interesting choice. Her motherly nature could be put to the test in the rather unorthodox Family Guy world. She might try to take care of the Griffin family, especially the kids, in her own way. Her interactions with Lois could be really interesting, as they are both strong female characters but with very different personalities. And her fire magic could come in handy when dealing with some of the more dangerous situations in Quahog.
Frisk could be a very interesting character to focus on. Frisk has the power of determination, which could be used in unique ways in the FNAF setting. They could befriend some of the animatronics, using their kindness and determination. Also, since Frisk is the player - controlled character in Undertale, it would be interesting to see how they adapt to the more dangerous and unpredictable world of FNAF.
I think Monika would be really interesting. In DDLC, she has that self - awareness and a certain level of power over her world. If she were in Undertale, her attempts to understand the very different rules of that world would be fascinating. She might try to use her powers to change things, but in Undertale, the concept of determination and the power of the player is very different. So seeing her interact with characters like Flowey, who also has some unique powers and knowledge, could lead to some really intense and thought - provoking scenes.
Sans is another great choice. His laid - back but extremely powerful nature in Undertale would create a lot of interesting dynamics in a Deltarune world. He could befriend or baffle the Deltarune characters. For example, his time - related abilities would be very different from anything in Deltarune, and it would be fascinating to see how he uses them in a crossover situation, like against the Darkners.
Sans is an obvious choice. His laid - back yet perceptive nature would make his interactions with the FNAF animatronics fascinating. He might be able to see through their mechanical exteriors to the tortured souls within. And his ability to teleport and use his 'Gaster Blasters' could play an interesting role when dealing with the animatronics' attacks.
Frisk and Dave Strider would be an interesting pairing. Frisk is all about kindness and determination in Undertale, while Dave is this cool, sarcastic guy from Homestuck. Their different personalities could lead to some really fun interactions, like Dave trying to be all cool while Frisk just goes about their business of being kind to everyone.
Blake and Brian could be an amazing duo. Blake, with her love for books and her complex past, and Brian, the intelligent dog who is also a bit of a philosopher. They could have deep conversations about ethics and morality, all while being in a world that is a mash - up of RWBY and Family Guy.
Perhaps it would involve the characters from Family Guy somehow getting trapped in the Underground of Undertale. Peter Griffin could try to befriend the monsters there, like Toriel, while Stewie might be intrigued by the unique magic system and try to use it for his own evil (or not so evil) plans. The different art styles and tones of the two shows would create an interesting contrast in the fanfic.
Ruby would be really interesting. Her innocent and positive attitude would contrast greatly with the dark and desolate Fallout world. She'd be like a ray of hope in the wasteland. Her scythe - fighting skills would also be unique and useful against the various threats in Fallout. She might try to befriend some of the mutants or ghouls, seeing them not as just monsters but as living beings who need help.
Sinister characters like Adam Taurus from RWBY could also be interesting in a Bloodborne crossover. His already dark and violent nature could be enhanced in the Bloodborne setting. He could form alliances with some of the more sinister forces in Bloodborne, or perhaps he becomes a sort of anti - hero, fighting against the greater evils in the Bloodborne world while still being a morally ambiguous figure.
I think Naruto and Stewie would be an interesting pair. Stewie is this super - intelligent, somewhat evil - minded baby, and Naruto is a powerful ninja with a big heart. They could have some really intense battles of wits, with Stewie trying to outsmart Naruto and Naruto using his ninja intuition to see through Stewie's plans.