Chica from FNAF could also be a great focus. In Undertale, she could befriend Papyrus. Papyrus, always eager to make new friends, might try to teach Chica how to make spaghetti. But Chica's insatiable hunger for pizza (and her more menacing side in FNAF) could lead to some comical and tense situations as she tries to resist her urges in the Underground.
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 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 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.
Frisk and Chica would also be a cool combination. Frisk is all about kindness and determination, and Chica is this animatronic with a bit of a goofy side. Frisk could try to understand Chica's motives and maybe even help her find a new purpose other than scaring people at night.
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.
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.
Definitely Wolverine and Spiderman. Wolverine's rough, no - nonsense attitude would contrast sharply with Spiderman's quippy and more light - hearted nature. Their fight scenes together would be amazing, with Wolverine's claws against Spiderman's webs. And they could also have some really interesting conversations about loyalty, as both are very loyal to their causes.
I think Iron Man and Bumblebee would be really interesting. Bumblebee is a fan - favorite Transformer, and his more light - hearted personality could contrast well with Iron Man's often - cocky demeanor. They could have some fun banter while also working together to solve problems. Also, Iron Man could help repair Bumblebee if he gets damaged, and Bumblebee could show Iron Man some cool Transformer - only maneuvers.
In this kind of crossover, Gaara could be a fascinating character to focus on. His ability to control sand could be used in really creative ways against the zombies, like creating barriers or using sand - based attacks. From 'The Walking Dead', Michonne would be great. Her katana - wielding skills are similar to the ninja sword skills in the Naruto world, and she could learn some new moves from the ninjas. Also, Naruto's teacher Jiraiya. His knowledge of various jutsus and his overall wisdom could be key in coming up with strategies to deal with both the zombies and any new threats that come from the crossover of the two worlds.
Definitely Percy Jackson himself. His ability to control water and his overall demigod powers would make for some really cool interactions in the Halo universe. He could be a bit of an outlier among the Spartans, but his courage and quick thinking would be assets. Also, Master Chief. His stoic and badass nature would contrast well with Percy's more impulsive and humorous side. When they team up, it would be an amazing duo to write about.