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.
Frisk could be an interesting focus. Frisk is used to a world full of monsters and having to make moral choices. If they were in the DDLC world, they would be in a completely different environment. Their determination could be put to the test in a new way, perhaps having to deal with the psychological issues that the DDLC characters have. And their ability to reset might have unforeseen consequences in the DDLC world, like maybe they accidentally reset a day that had important emotional developments for the DDLC characters.
One interesting plot could be that the main character from DDLC, say, the protagonist, gets transported into the Undertale universe. They could start in the Ruins and interact with Toriel. Since the DDLC character is used to a different kind of world, their reactions to the monsters and the unique magic in Undertale would be fun to explore. Maybe they could try to use their knowledge from DDLC to solve some of the problems in Undertale, like trying to understand the nature of EXP and LOVE in the context of their own experiences.
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.
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.
Some concepts involve crossing DDLC with classic literature. For example, a crossover with 'Pride and Prejudice'. Monika could be like Elizabeth Bennet, intelligent and strong - willed. The interactions between the DDLC characters would then be filled with the same kind of social intrigue and romantic tension as in the original novel.
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.
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 would also be a great character to focus on. If they were to gain some of Saitama's power, it would completely change their journey in the Underground. They could use their new strength to protect the monsters instead of just trying to find a way out. This would lead to new relationships and storylines. They might be able to stand up to Flowey in a different way, and even change the way the entire Underground views humans.
Ruby and Frisk would be a really interesting pairing. Ruby is full of energy and determination, and Frisk has that unwavering kindness. They could learn a lot from each other, especially in terms of how they approach difficult situations and make decisions.
I think Leonardo and Papyrus would make an interesting pairing. Leonardo's leadership and discipline could contrast with Papyrus' more naive and enthusiastic nature. They could learn from each other, like Leonardo teaching Papyrus some ninja moves, and Papyrus showing Leonardo how to have more fun. Another great pairing could be Donatello and Alphys. They are both the 'tech - smart' characters in their respective universes. They could collaborate on some crazy inventions that combine Undertale technology and Turtle - made gadgets.
I think Freddy Fazbear and Toriel would make an interesting pairing. Freddy is this big, imposing animatronic, while Toriel is a motherly figure. Their interactions could be really cool, like Toriel trying to understand what Freddy is and Freddy being a bit confused by Toriel's kindness.